I know this might sound crazy, but has anyone thought of any good reasons a player might side with Alduin? Anyone out there roleplaying such a character?

Can you even side with him? I can see that you could/would if you wanted to destroy the word like some power crazed person, I would just to see the affect though.

Well, greybeards ask the main player to think about if perhaps the world was meant to pass.
Although Alduin does a piss poor job of making passing on appear attractive.

Only reason to side with Alduin is wanting the entire world to end... The best way to go about that, is to simply let yourself die in any random fight. Boom Alduin has won.

It's funny but I got the impression Alduin was trapped in an endless cycle and only the one who could best him could end that for him. But as I said previously he does a piss poor job of communicating that if it were true. Whenever he opens his mouth and speaks, he comes across as another textbook villain.
Alduin often came across as one of these things...
http://tvtropes.org/pmwiki/pmwiki.php/Main/VillainByDefault

Yo, he eats worlds and is mean about. Kill him.

Obey the Dovahkiin or he eats your soul.

This, unfortunaly you need to reaload a save. Such a shame isn't it? Also Alduin was too weak. Akatosh demaned that your character banish him. Even if in the LORE terms he could kill all Sauron army from LOTR with a single Unreleating Force, Dovakhiin could move mountains as well.

Siding with Alduin is a better choice than siding with the Thalmor, with Alduin there will be a next Kalpa.