I had 2 alien bases for a long time (several months, I finally just raided one in Sept 2602). The score was easily manageable, and them bringing me supplies was nice. Main reason I waited so long was to get some better armor, I raided it with 2 suits of Juggernaut, 2 or 3 suits of Revenant, and the rest in either Guardian or Defender (well, and 1 nurse and 1 seductress), using the Metallo as the transport. Once a couple of troops recover, I'll probably pop the other one too. Luckily, the two bases are/were Church and Trade Guild, so that's a little less scary than it could be.