I'm not trying to be antagonistic, I'm genuinely curious: Why?
It seems like XP is a measure of how difficult a monster is. So is level to me; I can't figure out how I'd use them differently. Is it the scaling properties of XP (giving us an automatic rule for how monsters of different level combine in an "approximate budget")?
Is the difference that you want to use level as a measure of the monster's "tier" or "magnitude", as distinct from its actual difficulty to take down in a fight, or something?
Because I don't get it
Just because of the whole minion-standard-elite-dragon...err...solo thing. Some monsters are supposed to hold more weight in a fight than others...and that's a fight design thing that you want to have available at all levels. I'd be perfectly happy to drop the whole XP side of things entirely
if the systems could be worked in such a way so that you could simply use the Level & Weight-Class designations without more than a few "You can swap a
this for
that." rules.
Bounded Accuracy is great and all, I suspect it will smooth out the distinctions between minion-standard-elite, just through HP totals etc. However, Boss-monsters like Dragons will still need to be different. "Leader" type monsters, even if not elite, might also need some special consideration. I just don't see how, even with BA, a 10th Level Dragon will be a good "boss" for 4th level PCs
and a good "standard" for 10th level PCs
and a good minion for 15th level PCs. His abilities will be too few and powerful to make a good climactic fight for the 4th level PCs, or too many and too weak to make a good minion for the 15th level fight.
I am willing to be surprised, though.
I go back and forth on this myself!
An individual kobold is clearly not the equal of even a first level character; if you need 4 of them to be a "good challenge" for John Q Firstlevelfighter, then I have a hard time seeing that as anything other than a fractional level of 1/4.
<snippage>
Why do you need to rate them individually then? I have a hard time seeing a situation where its useful to say "Aha! If I just add one more little kobold, then this encounter will be perfect." (Even if some encounter-building systems tempt you to do so.) If it takes 4 of them to give a Level 1 PC a good challenge, why would any less than a multiple of 4 ever appear? The monster listing could just say "Kobold Scouts" and note that there are 4 of them. If there's a need for an individual Kobold (for story purposes or whatever), he might be a good candidate for a higher level. Better yet, he might be a candidate for the old "0-level" designation.
What is gained by having fractional monster levels? Once you've answered that, you have to ask if that gain is worth the extra repeated overhead of dealing with the fractions at (at least) the start of virtually every campaign that is every played. It seems to me that noting "these monsters come 4 at a go" here and there is a lot easier than several paragraphs of math than need to be constantly recalculated.