Hey now, dedicated servers have been asked for for YEARS on Halo. Personally, BF3's dedicated servers are awesome. Rarely do I face lag when I play the game, but in half, if not more, of the games I play on Halo, be it 3 or Reach, I face lag. I wouldn't want them to be join in progress in Halo (unless it was only set up in social gametypes and not any sort of ranked).
You don't see a problem with lag? Lucky you, others do and are entitled to their opinions.