Which is great so long as you know what level a spell is.
Which you probably do, because you've been conveniently browsing them /by level/.
If not, an alpha index of all spells wouldn't be too difficult.
If not that, either, then looking through the class's spell list works. And, you only have to do it once when looking up a single spell, vs flipping back and forth between the class spell list and the universal alphabetical list for each and every spell while trying to familiarize yourself with your choices.
Plus the putative 'all-in-one-list' advantage is lost the moment a new spell comes out in any source, anyway.
I remember 2nd Edition when monsters had spells, and I had to guess whether it was a wizard or cleric spell and then go through each spell level. Not fun and just slow at the table.
And in 1e, the MM referenced 0e spells! Yeah, classic D&D was not that user-friendly in a lot of ways. Even so, it was nice to be able to browse spells by class/level and have an idea what your character could do. Actually, that bit was kind of self-correcting, because you associated spells that you read close-together, learning which spell was which class and what level. So, if as AD&D expected, you were a very experienced player before taking up DMing you probably would know at a glance that
Fear is a 4th level Magic-user spell but
Bestow Fear is a 1st level Cleric spell that you'll find under the entry for
Remove Fear. Either way, though, still much more trouble than having all the monster's tricks right there in it's stat block.
Especially with overlap between classes.
That is an issue with the AD&D way of organizing spells "except as listed above and noted below, this is the same as the different-level/other-class spell of the same name..." annoying in exactly the same way as having all the spells listed alphabetically and flipping from the class list to the spell description, just only with shared spells. But, a real problem...
Admittedly since there's so much cross-over in spells between classes (*cough* Bard *cough*) there might be some difficulties with that, but I have a simple solution for that as well. Make the spell lists more differentiated between classes.
And that's a nice solution, indeed. Not only makes the listing easier to read, makes the classes better differentiated.