The point of multiple factions is mainly about identity. With a miniatures game it takes days and weeks and months (and sometimes years) to purchase, construct, model, and paint a full army. At the end of that, you want your army to have a unique point of view, strategy, and aesthetic. To that end, multiple factions make this easier and you get fans who ardently support their chosen faction ("I'm an Eldar player" or "I'm a Haqqislam player") and the lore behind it.
Yes, thank you. Most people on the FFG forums have never played traditional tabletop minis games and aren't really very aware of what they are like and how important faction identity is to the experience.
Historically, FFG has not understood this. They are new to minis games and sell them in an strange model where you have to buy models from other factions to get the pieces you need for your own. They have managed to get away with this because the games have 2 factions, or in the case of X-Wing they added a 3rd faction after everyone was accustomed to the model. But this does not scale to more factions. People aren't going to buy 3 or 4 or 5 factions worth of models to get cards for their own faction, they will instead just not play the game at all.
X-Wing barely gets by with three factions, and it had the advantage that you often only need to buy ONE model and you're done. Runewars won't have the advantage, because a full sized game there is not 3-4 models on the table as X-Wing often is. You'll need to buy a lot more models which means more cost and when you multiply that by more factions as well it quickly becomes unbearable for normal players.
So, it's going to be interesting to see what FFG does. Will they sell the game in a more conventional way? Will they sell card packs so you can get your upgrades without buying a ton of extra models? Will they make all the upgrades faction-specific? Or will they push the community too far and watch the game die on the vine?