You have to keep the alternative in mind when looking at that part of design. Drawing from my experiences, I can tell you that a lot of needlessly inconvenient things happen once you start forcing people to party to progress. Lineage II had the thing where it was really hard to solo a same-level mob once you passed lv30, and your entire partying experience was defined by what class you clicked on when you rolled your character (as archers, the nuke-based mages and any tank that was not a dark avenger were instantly LOL-worthy for partying). XI had that and an even worse problem in that aside from the tank and healer roles to fill in, you also had a "refresher" spot to fill.
Both suffered from travel time blues and spent more time than they should waiting and getting stuff together. Worse off, you couldn't really do something else while waiting because of travel distances and what was required to actually survive on your own (more true in XI). And that's just partying.
On the progression side, I'll give my two cents. I'm personally fine with not having the best of the best. I never had Excalibur or Murgleis, never had any of the legendaries in WoW, and I'm fine with that. That, however, doesn't mean I should be exempt from decent gear that the PvE encounters are balanced around. If the hardcores need something to stroke their e-peen about, there's always the option of hard-mode fights so that they get their jollies, so long as I get to see the content in some capacity.
SE learned this lesson with CoP's zones being a playground for the 1337 for as long as they were. Likewise, Blizzard learned this from all the people who were mad that only the top 2% got to see Illidan at all during TBC.