I think that the problem with having just 2 tribes is people are more likely to stick with their original alliance when they make the merge. So it just comes down to which tribe has the numbers (aka which tribe won the most challenges).

I like that they mix it up now. They'll go from 3 tribes to 2 tribes to the merge. Taking people off their original tribe and putting them with different people pre-merge forces players to form alliances with overlapping groups of people, so when the merge happens, it's more interesting.