I've heard various ideas such as limiting it only after mods have expended their points (this will require implementing a cooldown to prevent a user from getting points again too soon). I want to hear your feedback, and I'll roll together something for the next major update of the site. Leave your comments
I think the reason being unable to mod on a discussion where you've also posted is a concern is because post counts are still relatively small, making it easier to add to a discussion (feel more like you'll be heard) and discussions still need all the comments they can get... meaning LESS chance to spend mod points, because, well, *discussion*.
Second, I'd be more likely to spend all my mod points over 3 days than over 4 or 8 or 24 hours. I take it seriously and I only mod up (a post must be truly egregious before I'll mod down, and I think that's happened all of twice in 15 years). I feel a responsibility to use them well, not to use them fast, and that conflicts with their short shelf life.
Also, I think worrying over "reaching +5" is silly -- if anything, right now a lot of marginal and meh posts are winding up at +4 and +5, probably because the mod points expisre so fast that they're just being flung at the nearest halfway-intelligent comment, oft as not.
In short, I'm good with the slashdot system; obviously it worked very well for a long time, and survived a number of upheavals and shifts in the userbase without becoming either a burden or a drawback to discussion. Let nature take its course and don't try to micromanage, and all will go well.
(Been on the road 4 days and where do I come to catch up on the world? You guessed it. :)