This is kind of a pointless topic.
Give Sonic 2, is there any reason to play Sonic 1?
Given Super Mario Bros. 2, is there any reason to play Super Mario Bros. 1?
Given Resident Evil 4, is there any reason to play the previous entries?
Etc.
Oh, I considered these before posting! Mario 1 has significantly different gameplay from Mario 2 and 3, so it's absolutely still worth playing. In addition, all Sonic games have great level design, so they're all worth playing, despite having similar gameplay. Bonk's Revenge, however, maintains the same basic gameplay as Bonk's Adventure but with vastly superior level design.
Level design in Bonk 2 is only "vastly superior" in that it's just different.
Either way, my point was that there are countless instances in video game history (as well as movie, album, etc) where sequels 1-up their predecessors in one way or another. Doesn't always mean the original isn't worth bothering with, unless the original happens to be a steamy pile of crap, which Bonk's Adventure surely is not. It's a cult classic at the very worst, and has a style and charm absent from the entire rest of the series. All subsequent games (including the SFC entries) were based more or less on the mechanics and ideas of Bonk's Revenge and therefor derivative as such. Variety is the spice of life, and you can play Bonk's Adventure back to back with any of the other four "real" entries in the series (I don't count the NES or Gameboy versions; those were cheap cash-in attempts as far as I'm concerned) and you're guaranteed a fresh experience.
Having said all that, I agree with Necromancer wholeheartedly-- Bonk 3 is by far the best in the series. It took everything that worked well in the first two games, combined them, and threw in some new ideas of it's own. I'd say Adventure comes second, because it's so different and has that venerable charm. Bonk 2 is arguably the worst of the bunch, actually-- horrible spin control, recycles it's own levels with palette swaps and minor adjustments (1-1, 1-2 become 5-3 and 5-1 respectively), lackluster bosses (stage 5 is missing a boss altogether), etc.... Don't get me wrong-- I love all 3, but I'm just trying to prove a point. Revenge was a "concept game." Some of the new concepts worked out-- some not so much. But to say it was so grand it rendered part 1 obsolete-- preposterous.