Useful when it didn’t become the center of music. The real musical advances were when Eric Clapton cranked his Les Paul and Marshall combo to 11 and overdrove the vacuum tubes in his amp and inside the mixing board, and when Dave Davies cut his speaker cone with a razor blade on ‘You Really Got Me’.
I agree, like the way Rick Wakeman used it on David Bowie's Space Oddity, and later with Yes. Wakeman talked about how difficult it was to keep the Mellotron working though. It's probably not the kind of instrument that does well traveling from gig to gig.