I played quite a few and COH was the only one that I played that allowed it to happen. I have no clue about WOW I skipped it and went to Horizons before ending up at COH. I went back to Rift for a bit after COH went under but if you can not raid (which is hard for me to do anymore) then it is quite a boring game but you be running a 20 man group with one debuffer/buffer because all you did was over write each others debuffs. DAOC did not allow stacking debuffs but that had a lot of PVP in the realm wars(which was a big part of the game) so I think they were always scared of stacking debuffs and control powers lots of teeth gnashing over them because of the big PVP component. Ultimta really did not have alot of debuffs back when I played it but that was like 1997 and was a full open sandbox game uh the body runs. Few handful of other ones
It might come down to what MMOs a person has played and not played. Some MMOs use a system I tend to call "named buffs." I think Rift was like this and Guild Wars was like this. Buffs aren't stackable quantities because they are really more like booleans: you have them or you don't. You can't stack two of them because they are light switches: on or off. Some systems even went farther (I think Warhammer was like this) in that there were buff categories, and you could only have the best named buff within each category.
I think SWTOR originally allowed most things to stack, but I think they changed at on the second or third balancing pass. Eve Online allows stacking, but they (being probably among the most mathematically literate MMO developers I've seen) use what a CoH quant would call multiplicative stacking. I played both D&D Online and Age of Conan long enough to know both had some form of buff stacking in at least some cases, but not long enough to have figured out the byzantine rules for when it does and doesn't. And of course coming from Cryptic, CO and STO both have at least some form of buff stacking, STO with more limits than CO (I think: I haven't played CO in forever). I think DCUO started off with more buff stacking and gradually moved more to binary exclusive buffs.
Even when a game has exclusive (non-stacking) buffs, there's the separate question of how that impacts gameplay. For example, consider two buffers A and B. If they both possess buff X and X is non-stacking, then only one of them can really provide that buff: the other one cannot buff the team. But if buffers A and B both possess a range of buffs, say X, Y, and Z, and X, Y, and Z are all exclusive, it could still be that while the buffs don't stack, the
buffers can stack by each providing one of those three buffs to the team. The problem with stacking comes mainly when two different players (characters) both offer the exact same thing and only one of them can do anything at all. In other words, even in City of Heroes where almost all buffs (from different casters) stack, two FF defenders can still end up not really stacking with each other in practical terms. Meanwhile, in another game two identical buffers could stack even if their buffs don't stack, if they have a range of mutually beneficial buffs.