(3) Having agreed to do something silly (like wearing a uniform) may put you in a frame of mind where you're more likely to agree to other silly things the leader of the group asks you to do later.

Why are uniforms necessarily silly? Let's take military dress uniforms. In the US, you can tell a military member's rank and branch of service, and even get an idea of their service record, just by looking at their dress uniform. To insiders, this can be rapidly gleaned looking at someone from across a room. With millions of members, individuals cannot possibly be expected to know everybody else and so the uniform serves a useful function.