I'll admit it: I play more than one army in WarmaHordes. It starts simple: "Well, I've got a couple armies up and running. Hey, a battle box won't break the bank.."
Then, it's "Well, I'd like to play 15, so I'll just get a solo or two."
"I'd like to play 25. I think I can get a unit. Maybe I'll get some mercs! Then I can use them with multiple factions. It's economical!"
"Hmm. I have all these merc solos and units, if I get a merc battle box I've got ANOTHER diversion..." And before you know it, BLAM! Multiple armies.
Is this good or bad? You decide. I think it's a little bit of both.
The Good
The bright side of collecting multiple factions is that you get variety, pure and simple. There's one flavor of variety in switching out your warcasters. Most of the time, it involves tweaking the army, and occasionally it's a radical redesign (I mean, try swapping Terminus for Denegrha, and adjusting for the difference in warjack points. Probably not gonna work so well...).
Gameplay
The bright side of picking up a new faction (or maintaining an alt faction or seven) is that it forces you to learn a new way to play. Epic Butcher and Epic Asphyxious have very different ideas about how to win a fight (hint: one is 'AXE TO FACE!' and one is 'I'll be hiding over here now, lettming my army do the work'. Hint 2: Butcher has an axe.) AND access to different options.
How's this a benefit? Well, honestly, I think that any intellectual exercise is good exercise. You apply some problem-solving and study skills* in order to grasp a faction's pros/cons.
It also helps you figure out how these armies play, obviously. If you've played Terminus a dozen or so times, and Terminus plops down across the table from you, then you've got a pretty decent idea on what to expect. Same goes if you see the Harbinger, or even the less-flashy casters. You'll also be able to look at his units and have a pretty solid idea of what you can get out of them, because you've tried to get the most out of them yourself.
Hobby
Plain and simple: variety in what you can paint and model. Tired of painting up a bunch of faceless stormknights and doing electricity effects, and want to paint fur and flesh? Look into Circle Orboros. Sick of the undead, and want to move to the draconic? Ditch Cryx, get Everblight. Want a wide palette of stuff that doesn't necessarily have to be uniform? Mercs have an opening for you.
Basically, you want an army to have a unified look on the field (even if it's just having the same bases) and that imposes some limitations on how you paint. A different army can give you a complete 180 in what you're painting, so if you're sick of doing armor, you can drop the Storm Knights and pick up some Tharn Ravagers.
Community
You can inject some variety into your community. How important that is, is up to you. We had one guy who was a good sport and, literally, out of the goodness of his heart, picked up Menoth for our local group's map campaign.
Additionally, if you're going to do demos, it probably helps to show off more than one army, even if it's something like a battle box and solo(s).
The Bad
Money
Let's start with the obvious low-hanging fruit. Picking up a new warcaster means you'll want new things for your army. That's an expense. However, you can probably use a lot of the support, and they may still be able to use some of your existing stock of jacks/units. Picking up a new army, on the other hand, means investing in new support as well, and a new book, and possibly more paint/flock to flesh it out.
The bottom line? You pay for the variety.
Time
Part of the joy of having a variety of factions is being able to switch. Well, time's a switching cost as well. It takes time to get good with your chosen faction, and if you take a radical shift (IE: Khador is your prime, and you pick up Circle Orboros) you're going to have a bit of a learning curve on top of the usual. This also means more time studying/practicing in order to get as good. It also takes time to, y'know, assemble and paint stuff.
Gameplay
This deserves a mention outside of time. It takes work to learn your faction. It's a form of specialization; you become accustomed to how your units perform, what your jacks can do, and your support options. You get used to fitting those into a warcaster's capabilities, and then you learn how all of those interact with the scenarios and other people's armies.
And then, well, you change your tools and that can change EVERYTHING. It's like spending every day for a month at the range learning to fire a .22 pistol, and then deciding you'll go to a target match with that brand-new high-caliber rifle you bought. Sure, there's some commonality between the two, but your muscle memory and well-practiced habits don't quite fit with what you're holding.
Basically, you have to be willing to take the hit in your gameplay. Learning to lose gracefully should be a prerequisite for this hobby, and making drastic switches is a good way to get more practice in it.
Wrapup
Honestly, I'm a fan of collecting multiple factions. I'll freely admit it's a bit of a reaction to my years as a Warhammer 40k player; there were some serious switching costs (expensive models, the requirement for lots of models, the lack of bargain-priced starting packs, and the game's inability to scale well at all to lower point values are factors). WarmaHordes is significantly more affordable in that regard.
That aside, there are really only two questions that dictate whether or not multiple factions are for you:
1) Do you want to branch out and play other factions? Honestly, if the answer's 'No', it's fine. It's not necessarily for everyone.
2) Do you have the time/money to put towards it? It's not cheap, and it wants a little more of your time than just one faction will in order to get your mileage out of it.
If you answered 'yes' to both of the above, then you'll get to reap the rewards. Me? I've got no family and am currently flying solo**, and I enjoy multiple factions. It's strictly a personal preference thing.
And if that picture made you nostaligic for some Christopher Watkin dancing, here's the obligatory link to it.
*Yes. Yes, I did just go there. Seriously, tell me you can win against rouger opponents without having an idea of what everything does, and how it fits together. Game really DOES require some problem-solving skills, logic, and analytical thinking.
**Brief soap box moment: it's a statement of fact in my case. I'll wax poetic on life for long enough to tell you that there are two things in life: 1) What happens, and 2) how you react to #1. If you can't change #1, change #2, and you're probably going to be at least semi-content. Also, not all gamers are single; some are married, and some have families. That would tend to cut into your spare time...
