Variety? Some people might like to change up the schedule instead of spending 4 days at in one location. Others like to stay put, it was suggested as an alternative with its own pros and cons. I've done 4 hour trips to and back from dive sites that I didn't particularly enjoy, so I'd rather do day trips out of phi phi/krabi/lanta over phuket if i couldn't do a similan live aboard, mostly because I prefer other places over phuket, but to each their own. Or we not allowed to offer alternatives on this board for someone to consider?
Watboy, what would a day travel to Lanta or Phi Phi add for the op? They can dive Similans etc. from Khao Lak, and all the other sites mentioned here (like Shark point area, Hin Daeng, etc.) on daytrips from Phuket. It makes much more sense to accept the slightly longer travel time to a divesite from Phuket over a days travel first to save maybe 2 hours travel time the next day. Plus they'd also have to come back.