Is there any good reason for choosing spears as your weapon proficiency?

I never use them - so no ;)

If you do not mind sacrificing a shield slot to be able to dish out the second highest damage in the game possible... then sure, by all means use spears.

The game is plenty of magical spears, even a very powerful one, (The Impaler, Thac0 +3, damage + 10, I guess). But i find they lack secondary effects, my favorites (ex, the Casomir that grants 50% magic resistance, Celestial Fury, paralises enemies, etc). I never pick then.