Any real-life gun instructor will warn you against shooting traffic signs and garage doors, much less a contoured steel plate. You'll shoot your eye out, kid.
I would like to point out that 'steel' like we have now and plate armor from the middle ages aren't of the same quality. Also keep in mind that early on armor wasn't made to withstand bullets (or arrows/quarrels) from short range, this eventually changed. But I seriously doubt that firearms have insinuated themselves so much in D&D society that the plate mail is bulletproof. Look it up, wikipedia mentions it, they talk about short range. The first increment for pistols is 20ft and after that there are serious penalties, -2 for each additional 20ft. Also, a real-life gun instructor will warn you against shooting people and pets (aka. Monsters), so how much that impacts your gunslinger... ;-)
This is also the game where when you aim for the head, you hit the head, you only do a (little) damage and might stun someone for a round. So reality might be far fetched for RPGs anyway ;-)
Also, why a specific Ninja/Samurai class? Simple there's a lot of power in a name, Ninja/Samurai are iconic for asian (fantasy) RPGs. Just using the Rogue/Cavalier class, and calling them Ninja/Samurai doesn't do 'it' for a lot of people. You could ask yourself, why do we need a Ninja/Samurai class, simple, because a lot of folks do want it because their iconic for asian (fantasy) settings. Also, their not filling a 256 page book with these three classes, it's a relatively small section, if they fill 10% of the book with these classes (and directly related material) I'll be greatly surprised.
If you don't like them on principle, don't play (test) them. If you don't see the need for separate classes, again, these classes are not for you.