Among all of the arguments of whether or not Ninjas and Thieves are 'similar enough' we're ignoring a major point here:
No one has even tried out Rouge/Ninja yet.
While there are concerns about players who don't want a magic caster as a scout class, there's no real clarification on just how 'ninjitsu' reliant ninjas actually are. At this point, I think dagger lovers are jumping the gun. (Cause, you know, Rouge/Ninja did manage to jump in before Musketeer, right? Ok, I know. I'm fired...)
All I am going to really get into on this matter is: Give Ninja a shot. If you don't like it, you don't like it. But nothing really should prevent you from sampling the class/job, really. Thayos made a fairly strong point earlier in a different thread: We seem to be trying really hard to pigeonhole our experiences into preset expectations with little to no flexibility. That sort of habit really does make it hard to enjoy much of anything.