I don't really mind, they can work with either STR or DEX, but what I want to focus is about the part whether it seem to be a debuff + DPS or just a pure DPS.

In any MMO, you can only become the best at one thing. Any hybrid class will never become the top of line DPS. So if Ninja can deal both spell damage and physical damage, then he is going to be bad at both. If he focus on just one type of damage, then he may get chance to be the top. And in this sense, I think that if it is confirm that Ninjutsu will debuff enemy, then I can totally see that it will be the class opposite to bard with debuff instead of buff. Overall dps will be weaker than pure DPS class such as Monk for sure.