Diffusion's recast prevents you from being any more of a buffer than you were before, so it's effectively useless. Since there are jobs that have spells or abilities that are roughly equal to or greater than anything a BLU has, Diffusion is a novelty at best, a wasted merit that you'll use just because you want to at worst.
That said, if the recast were made to be like Feint's, the uniqueness of BLU buffs would be of greater consequence and the job could actually be used as a true buffer.
Assimilation is probably the most practical Group2 to merit, next being Convergence, if you regularly nuke as some BLUs do. Some people think Enchainment is good, but I don't think it'll be worth it until they boost G2 upgrades to 5. At any rate, Diffusion is pretty much the worst since you'll only be using it 5 times an hour for something that most jobs can either do themselves or get more regularly from another job.
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