If you're never gonna put any points into intelligence, should you bother with willpower since you're not gonna have enough magicka for later spells? I think I'm regretting about choosing the lady as my birthsign.

Depends, really. Do you intend on spamming lots and lots of low magicka spells? If so, then yes -- willpower is a good investment. If not, eh....probably not.

I do spam my low heal spell a lot... along with my low fireball, and my feather spell. I only have 88 magicka points, and my willpower is currently level 47. I don't see any points in increasing willpower. Does it increase the damage or anything?

Don't think it increases damage, but it -does- increase your magicka regen -and- your fatigue. That's why it's good for spamming low magicka cost stuff.

It will increase fatigue. which can be helpful to a melee character
http://uesp.net/wiki/Oblivion:Fatigue

Ooh, so it recharges your mana quicker. I guess that does balance everything out.