- Thieves make it so that other classes can't do what they do. Before Thieves came along, all the characters had to pull their weight doing thiefly things like disarming traps and picking locks and pockets.
- Thieves can infringe on the proper role of fighting men.
Both of these objections have to do with our thiefly class usurping another class' role. The first objection is mostly about things that are stereotypical of thieves, like picking locks, and the second has to do with being so good at fighting that a fighting man gets serious competition.
The first objection is, honestly, the objection that makes me most open to leaving out a thief class in my games. I don't want any particular class to become absolutely necessary to a party, nor do I want the existence of one class to mean that other classes become more limited than they were. If I have to choose between using a thief class and preventing these things from happening, so long thief class!
I'm not convinced that I have to make this decision, though. Here's how I think my Scoundrel can address this objection:
The Scoundrel is going to be made up of numerous options that can be chosen by a player, a la Telecanter's Choose You Own Rogue, and some of them are going to be abilities and some are going to be skills.