Hiya!
TL;DR
I tried the "just level up whenever it seem appropriate" a couple of times over the decades (yes, it was a 'thing' spoken about in hushed voices in the dark recesses of the FLGS where the DM's gather monthly under the shadow of a new moon...
but you didn't hear that from me! I would never break the sacred Blood Bond of the Ones Who Walk the Shadows! ).
Ahem...uh, so yeah. Tried it. Each time it felt...'wrong' for AD&D/D&D. Other games with XP...never bothered me. But for some reason, probably because D&D was my first time partaking of the forbidden fruit, XP and level advancement just kinda needed each other. Like sushi and wassabi, or New Years day and hang-overs. I wanted to get away from it multiple times but it keeps drawing me (and my players) back. To us, I guess it's like "gold pieces for the Players"...it's just something that they can gauge their general success at playing.
Sure the story is cool, and the laughs, and the unexpected happenings throughout a good session, and all that other stuff that comes with all RPG's of any worth. But playing a 1st level Elf in BECMI, surviving on wits, luck and your companions, fighting veritable hoards of goblins, kobolds and giant rats, taking on Cave K and defeating the evil clerics, slowly gathering your coins and treasures...and then, at the end of a session in a long line of sessions, when your Elf is enjoying watching his friends and compatriots celebrate a hard-won victory at The Inn at the Keep, the DM speaks: "Silverleaf, you have gained 902 xp's". At that moment. That instant. When you look at your character sheet and realize you now have 4,016 xp's. At that moment, right then and there? ...well, that feeling of accomplishment is something that will
never get old. As a DM, when I see that "new level glow" surround the player's smiling face...all I can do is slowly, and proudly, clap and say "
Well done, sir...well done".
It's the gaining of those numbers, adding up 27xp here, 104xp there, another 74xp from that, and slowly but surely seeing a characters XP total rise closer and closer to that coveted Next Level is...well...it's just durn right satisfyin'!
As a DM, the now-known-as-Milestone-Advancement always fell...cheap? I don't know...kind of like I was just deciding "You take...24 points of damage...because it seems right", and then the character dieing because he's only a 1st level Elf with 6 hp's. *shrug* I'm a crotchety old grognard that is fairly set in his ways with regards to the whole "D&D should be played
this way..." type of thing. I don't begrudge any other fellow DM's who decide to use the milestone approach or some other method...each DM is god/goddess of their own world and Master/Mistress of their own game. That's cool. But for me (and my players), we like gaining XP from adventuring. I give less XP per monster in 5e ( about half or lower), but I give out 1:1 xp from GP's recovered or earned. Keeps players from focusing on just killing everything that moves.
^_^
Paul L. Ming