I am happy to see this change. This very day I was in a behest, and I'm telling you, it was tedious. Everyone but the Conjurers hit 1 for about 2 seconds, and the beast is dead. On to the next one, same procedure. This is because a full party is way too large, when nobody even has to use abilities other than the default class attack. Besides, it gets crowded, it's hard to get a good view of the battlefield.

So yes, a smaller max party size is definitely a change for the better.
As for those who fear all members of a Linkshell can't join - I see your point. But where the boundary goes will vary with each linkshell and party; small casual linkshells won't have this problem, and larger ones probably will, anyway. Swap your turns - don't bring along the same people everytime, and everyone will get a share of the cake.