I've had success with and without the Lair. I play without now because it fits my play style better. The Lair is a major investment, and it commits you pretty heavily to playing creatures, but it also frees you up to focus your mage's actions on other things. Quick Summoning mostly lets the Beastmaster be more free form with his actions, whereas Lair gives you a direct action advantage.
Low level swarms can be good, but you need a more durable threat for them to support. Fast swarms, like playing 6 Bitterwood Foxes in the first 3 turns are incredibly vulnerable to any number of counters: guards, zone attacks, Chain Lightning, Mordok's Obelisk, Suppression Orb, Suppression Cloak, damage barriers, sweeping, Whirling Strike... Some of these can be mitigated to some extent by careful positioning, but I think those swarms are too vulnerable to be the basis for your play by themselves.
Swarming once you already have a durable threat or 2 is much more viable. The other thing is that the BM's Quick Summoning allows you to swarm while also attacking with your mage. Maybe a Bitterwood Fox can't stand up to a Knight of Westlock or Necropian Vampiress, and another mage can't justify spending an action playing a level 1 creature in many high pressure situations. The BM can use his quickcast for the creature, and still have his regular activation to attack, which means that beatdown does not preclude swarming, even without a spawnpoint. This means that I can potentially play a creature every turn while still attacking. If you take the time to kill my creatures one by one, they're effectively serving as a block, and with Ring of Beasts, the mana is the same for a Bitterwood Fox anyway (although Block can be cheaper with a discount ring). The advantage they have over block in that regard is that attacks may sometimes whiff, in which case I could get a 3 dice attack in and force another attack to deal with my creature.
If you don't kill my creatures as they get played, I will build up, which will eventually leave me more vulnerable to those swarm counters, but I'm getting attacks in with those creatures as I'm building up the swarm. Even if you deal with the swarm, you still need to contend with my more durable threats anyway.
I like to open with a Steelclaw Grizzly on turn 1 or 2, and then consider transitioning into swarm, beatdown or (more commonly) some hybrid between the 2 depending on what seems to make sense. What I'll typically do is buff myself and summon low level creatures as I fight, particularly if I manage to get Fellella out to keep up the buffs as I summon.
If you suspect some of the stronger swarm counters, just stop playing small creatures after a certain point. Don't have more than 5 creatures in play at a time if you strongly suspect a Mordok's Obelisk or Suppression Orb. As your opponent plays more creatures, you can feel more comfortable playing with more creatures out, but it's just a question of avoiding over committing.
It's similar to a big creature and stacking enchants on it. Lord of Fire is incredibly threatening, but if you stack Bear Strength, Falcon Precision, Vampirism, and Cheetah Speed on him, it just becomes too efficient to use Banish, Turn to Stone, or Force Hold, and you've over committed to your big creature. Avoid giving your opponent the opportunity to play efficient counters, particularly if it's your only threat.
