Comment made by: mikera
I think it may be important if you have "sequence-like" data structures that aren't precisely sequential? but are conceptually equivalent to sequences. My custom Expression type is one example, and for interop reasons things like java.util.ArrayList spring to mind.
As it happens, I've switched back to using regular lists for the moment so the issue isn't a blocker for me. But it may still be worth thinking about.
Couple of advantages of doing this kind of unification would appear to be:
a) notational - you can use regular Clojure lists and vectors for unifying with something sequence-like
b) efficiency - avoid constructing a new custom object when it isn't needed (though the cost is probably too trivial to bother about in most cases....)
Of course, you may decide it is simpler and purer to avoid these complications, which is fine. But it seems a shame to have all the nice extensible protocols, and not quite be able to fully extend the functionality to custom types....