404 CP 98% IVs Jigglypuff

I've read that Pokemon below 1500 CP are not influenced by the rapid CP decay. Furthermore, people often attack with the big generalist Dragonite. If your wiggly gets dazzling geam it will be a solid defender and time consuming for the opponent.

No harm in evolving it, but I wouldn't power it up. The only possible use would be a Dragonite attacker, but there are better options for that.

I've got a 98 and a 100, and both just sit in my storage taking up space. I think I used them once. Still, no harm in evolving. Even if you later find a L30, it still won't be relevant.

Evolved up to a Wigglytuff (if you have the candies) isn't a bad idea for a Defender. Given how the gym mechanics work now, Wigglytuff is useful as a defender - you only need to hold a gym for 9 hours before it's overkill. And Wigglytuff is one of those Mon's that are tough enough to make filthy casuals (like FandangoGhandi) walk on and not try to take down a gym that is less than 9 hours old. But it's weak enough that it might be seen as a challenge by more hardcore players, so they might try to take it down.

I keep meaning to use Wigglytuff on gyms, but I'm having too much fun with my Wobbuffet that has Counter as it's Quick Move. It seems to last far longer than a CP 600ish mon should last on a gym.