The biggest problem is exactly that. That the main character only stands around and nods, making them a dullard without any kind of attractive and stage presence.
This, by the way, doesn't solve the continuity problem at all, as the other characters talk to the main character, call him by name, interact directly with him. The introduction of other adventurers would require even more generic (and thus boring) dialogue.
Mind you, even if they introduced a feature like that, it still would run right into the wall of the continuty problem. Even if you brought a full group of 8 into each story cutscene with everyone nodding and prancing around like brainless dullards, with the NPCs delivering some ranmdom motivational speech to them all, it wouldn't solve the fact that there are thousands of other groups of 8 around, tho which exactly the same facts happen, exactly the same words are told, by exactly the same people.
The story would still happen in a vacuum, only a slightly bigger one.
On the other hand, a story happening in a complete vacuum gives the developer a MUCH wider storytelling freedom.
Mind you, it's not even incompatible with having other people involved, as in the screen of each platyer he can appear as the main "acting" character, and the others can be the nodding dullards.
They can give players the option to chose beteween a range of personalities, as we already do with path companions.
The "flat dullard" personality, is *by itself* a personality. It already acts in a way that's "wholly indipendent" from the role anyone of us would like to assign to his character, and is in no way safe.
The fact that quite a lot of people have looked at their character acting like spineless idiots and wondered why the hell he was acting like that is the whole reason why threads like this (it's in no ways the first) exist.
the "spineless random dullard" is in no way more appealing or more generic than the "hero", it's just more unpleasant.
I'm quite sure that the vast majority of players would rather like to see his character act in pretty much any other way than as a random dullard weakling with no redeeming point. Give people options, and the risk of having the character act in a way the player won't like is *drastically reduced*, not increased.