Not convinced? Well, here's some numbers. Running a Timid nature and 252 Speed, Swoobat hits 359 Speed. That's just faster than Gengar, Archeops, Infernape, Mismagius, and other fast Pokes. It does fall just short of Timid Alakazam and Sceptile, and Jolly Dugtrio, Swellow, and Electrode though, so watch out.
After one Calm Mind, a Timid 252 Special Attack Swoobat reaches a jaw-dropping 556 Special Attack and acceptable 298 Special Defense. Suddenly, Swoobat has gone from a joke to a huge threat right before your eyes.
Calm Mind is obviously a crucial move on this set, as without it, Swoobat really is next to useless.
Stored Power works fantastically alongside Simple and Calm Mind. After a single Calm Mind, it reaches 150 base power (factoring in STAB), which is stronger than Psychic (135 after STAB). If you manage to get off 2 Calm Minds, Stored Power hits an incredible 270 base power, which is on par with a Victini's V-Create in terms of Base Power. Yeah, your opponent will stop laughing at your silly bat when his or her Pokes start dropping like flies.
Air Slash is the secondary attack move here, hitting Dark types that laugh at your Stored Powers. It's also Swoobat's best shot if it doesn't have time to Calm Mind, though you should really consider switching before attacking if that happens.
For the final moveslot, you can go with Roost for recovery, Heat Wave (B2W2 tutor) for damage and coverage, Hidden Power Fighting to smash Dark Types harder, Substitute to block status, or Taunt to stop Pokes from phazing you out.
One thing that's very important is spin support, as rocks can take a good chunk of his health right off the bat (see what I did there?). I'm still debating what items work best, Life Orb is certainly good, he's frail enough that the recoil probably won't make much of a difference. I've been thinking of trying Focus Sash though, to give him some defense against most attacks (notably Sucker Punch) that would OHKO him.
Move tutors in BW2 have been a blessing, in addition to Heat Wave he also gets Signal Beam and Giga Drain, the latter of which wrecks Rock/Ground types that threaten him with Stone Edge and provides him with an alternate form of recovery.
Even still, he has issues with coverage, especially against Dark/Psychic types like Houndoom, Honchkrow and Xatu, which are resistant/immune to Stored Power and his other options generally hit neutral at best. I think HP Rock is an alternative to Fighting, but it really depends on what you come up against.
Other than those weaknesses, Swoobat is amazing, and you should have one.
Btw, I know I'm not a pokemon professor, but I can post this if I want lol. I just love lower tiers, and my RU team is hard to beat