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
Complicated to code because of the way slash handles comments/parenting unless I ping the database a shit ton. That being said, changing the behavior to restore mod points may be the way to go here.
This will require a behavioral logic that after X period of time, points are either not refunded, or rollbacks don't occur to prevent people from being able to get modpoints "on demand" so to speak.
++, figure we can cache the value if need be in the database. We can write a slashd check to generate them retroactively by walking the parent on every comment to generate a thread id (which we can pin out somewhere on the comment bar). A good way to limit moderation to specific threads.
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. :)