Considering I haven't actually played any DQ game besides Monsters 1, one of the Monsters 2, and the first two games... are you surprised that most don't know they have names? That's the point being made. I am wrong, but it's very hard to know that. They aren't as recognizable and people are still calling them generic heroes as if they don't have names. That's a big deal for the character factor. The overall point is Erdrick is the most recognizable one by far, and he still falls overall to Slime.
Also, it's kind of silly to say Slime can't represent the series just as well when he can do practically all the same stuff every class can do. That's a non-argument towards him. They both have equivalent moveset potential. And yes, Slime can summon monsters too. They were always on equal levels of representing how Dragon Quest plays. They have the same overall gameplay in the regular style(of which DQ Monsters always was. Though I haven't played Joker, so I don't know if the general battle system is the same). I know little of Rocket Slime other than it being a spin-off.
Though as I said before, Erdrick isn't comparable to a lot of other franchise representatives because they use the bigger character first. Slime is just plain bigger than Erdrick. Slime also wouldn't be something as low as a stage appearance. If he isn't playable and Erdrick is, he's clearly going to be an AT or nothing more than a Spirit(cause no way he wouldn't get a Trophy. Being the mascot and all). I imagine the other Heroes and a few notable ones(like Jessica and Bianca) might get trophies too. And by trophies, I mean spirits, heh. ...Yes, I'm still a little salty Trophies got removed. >.<
Only major thing I asked anyway is if characters besides Slime got a spin-off. On another note, I made a Slime moveset that does the series a lot of justice too. It shows off everything you expect, including the usual weaponized moves you'd expect from a Hero, summoning monsters, and the usual spells(like Fireball/Hurt, depending what you want to call it).
Anyway, can the Hero be other classes? I legitimately want to know this. I don't have as vast knowledge as you do, so I asked a lot of stuff so I could be enlightened.