Excellent question and you are correct it very much depends on the opponent mage strategy. Whether I "react" in a sense to a more aggressive mage that isn't allowing me to build up or not the Lair is still critical to offensive or defensive play.
If you want to play a more offensive (creature based) Beastmaster the lair allows you to summon 2 (even 3) creatures. There is your "swarm" approach.
If you want to play a more offensive (melee/ranged based) Beastmaster the lair allows you to still summon creatures while your mage deals damage with a weapon or bow. This is a more balanced approach and what I was referring to, it is both mana efficient and you are aggressive (with your attacks) AND the creature support is vital end game...which again I use as a "balance" some defend me some attack, either way it puts pressure on the opposing mage.
If you want to play a more defensive Beastmaster the lair is still good because you can summon an extra creature and hide out by your lair. If I am playing this route (and the enemy mage isn't running AoE damage spells heavily) I do a lair/tree/fountain set up so there is lots of regen, healing, defensible support. This approach has won but it is slower and more boring, so I go with the previous method fight with my beastmaster and summon with my lair.
So if you want to go truly aggressive you almost need a Lair. There is a BIG difference between "rush aggro" and "aggressive". Sure you lose out on one round of mana/summoning to get that lair out, but it is more "aggressive" to use your full action to either ATTACK OR summon ANOTHER creature. Using your full action to summon a creature is useful "sometimes" but long term I feel that slows you down and is actually less aggressive. Don't waste a FULL ACTION is all I'm saying, aggressive or defensive extra Actions per turn win games. You are more in control.
The last 2 games I played, as I mentioned, I not only not waste a single full action summoning a creature, I also did not take a SINGLE point of damage to my mage. With my lair summoning support (to attack or defend me) AND me shooting or melee attacking they really had no choice but to react and eventually get overwhelmed. So it can still work if you are a fan of an aggressive Beastmaster IMO.