i guess you use what you have, and don't use what you don't have, but i don't really know when i've ever thought let me see what happened after that branch was merged into future.
I do have a simple script that will list what branches were merged into the current branch in git. That is useful, so being built in, in Fossil is useful i guess.
My gripe with Fossil, is not technical, it is really just who uses it? How much support does it get. When will it wither and die? Maybe that is an invalid concern on my part. But that is way more important to me than any technical issues raised here.