This is like the difference between "dead" and "mostly dead". It isn't much difference in some ways, but huge in its implications.

Anyway, it is at the confluence of the two points that you can get some reasonable milage out of an XP pool mechanic, even with those limits.
If your XP pool is 10,000 XP, and it is divided up among 10 "encounters" roughly evenly, then where it can be useful is giving the DM a rough idea of it being balanced--when the default conditions are met. Ideally, you'd like those default conditions set well away from the extremes--all creatures at once or all 10 groups separate. So say that the default presumption is that you'll have 3-5 encounters, of no more than three of the "encounter groups" at once. If you stick fairly close to the default conditions, you'll get something close to "fair fight" most of the time. If you crowds the creatures into fewer fights, it will be tougher. If you spread them out, it will be easier.
Where I think you are correct is that such a system is going to do a fairly poor job of telling you how much easier or tougher for X amount of crowding or spreading. That is going to depend so much on circumstances, resources, exact party mix, etc. as to make the variables too much to manage. However, that makes such an "adventure budget" system limited, not entirely useless.
What individual groups do about express crowding or grouping is where table preferences need to enter, anyway. Perhaps the amount if fixed, earn it best you can (divide and conquer is a prime goal). Alternately, perhaps easy or hard fights get an XP adjustment. Maybe the DM simply eyeballs it, with an eye towards maintaining the pace the group wants--"Man, that was a little harder than I expected. That puts us just a bit shy of 8th level. So what the heck, just bump 'em on up."