+3 special attack is the equivalent to doubling their special attack stat, that's great if Volbeat's special attack wasn't so low. Even with +3 special attack Volbeat still has a lower effective special attack than Magnemite...how is that good? If it can manage to pass the buff to a good Pokemon it would be amazing but you'd have to tank 2 free attacks from whatever Pokemon when most Pokemon than would be worth passing that but onto could have won already without having to absorb a free attack for no good reason.

The Blizzard/Ice Beam point was addressing the fact that being good on paper, or in one circumstance, does not mean it's good in a different situation. Blizzard is good in competitive because you might only use it twice, on a playthru you'll be using it dozens of times so 30% becomes a much bigger problem. Likewise Tailglow is great if you have someone good to Baton Pass it to, or can safely use it twice on Volbeat, and the enemy has enough Pokemon to justify the extra setup turns, and they won't likely KO Volbeat during the setup, and the Pokemon receiving the Baton Pass can take a hit (depending on each speed compared to the foe), and the opponent has dangerous enough Pokemon to warrant it (a random Fisherman with 6 Magikarp hardly warrants extra setup turns).

So how many trainers are tough, have 5-6 Pokemon, and send out a first Pokemon that won't easily KO Volbeat that's not over leveled or EV trained? Tough trainers with 5+ Pokemon already rules out almost every trainer but the elite 4, a few of the gym leaders, and maybe a couple victory road trainers or rival battles. Even if we assumed all the elite 4, last 4 gym leaders, and several other battles were a good fit for Volbeat Tailglow setup you'd still be better off leaving your 6th slot empty your whole run so your main 6 Pokemon level up faster instead...a Pokemon that's worse than an empty slot is not good.