>>21255546Point is that if you can only have one mega, there's really very little reason to not go for the best.
If you like a pokemon, or if a pokemon is below average but still fits in your team to fill a niche, you can justify fitting it in a team of 6.
If you can only fit one mega, you can't justify a shitty mega taking up the mega slot because you've only got one. You're probably better off switching mega beedrill with scolipede/Nidoking/Heracross whatever, so you can use the mega slot for an actually good mega.
Now if there were two slots, I'd say, Mega X is my prime mega, Mega Beedrill is my backup ass kicker.
That said, I suppose it might be possible for Beedrill to be good enough that he totally justifies the slot, but I just don't see it happening. Odds are there will be one mega that outclasses it, in either speed/power/coverage/defenses/etc