First off, Sweet Tooth coms from the oldest and longest running Playstation exclusive franchise there is, and he's the most iconic main character, there is no way in hell he is a filler. Radec represents the most popular shooter franchise on playstation-Killzone. While the character itself isn't popular, most people know of the Helghast as an enemy, and it is pretty clear upon first sight that Radec is a Helghast leader. He represents a really popular playstation game and he is instantly recognisable as the games popular antagonists. Emmett...i'll give you that, but he's FREE DLC and wasnt part of the launch roster, so we can hardly complain.
Anyway regarding your main point, i don't think it's worth a full priced game, no. I don't regret buying it full priced, but given the content of the game it's more of a budget priced game, which it is now, you can get it pretty cheap most places. To anyone who is skeptical i'd reccomened them to buy it cheap but full price is only for the big fans who were really excited, because the game just doesnt have enough in it, and the production values can be pretty lazy, but still, it's a fun game and honestly i have no problems with the roster, they got all the important characters they could, the rest is all third party agreements and such, although i do have to agree Raiden and Big Daddy are a bit strange.
sorry, but Sly isn't new. He's being RESURECTED. much different. And, Sweet Tooth is a PS1 LEGEND. Sweet Tooth, (Tell me if I'm wrong, I'm not an expert on this subject,) came before Kratos. Also, this game celebrates the best of the best in playstation. Sure, they couldnt get some of the obvious characters, but that's because they no longer own them. Also, Fat Princess sold really good on PSN.