Weapon Kata -- No, no ranks in the weapon are required, only ranks in the MA that it is being used with.
Yes, those are some of the major benefits (along with doing the weapon's crit rather than the MA crit)
Rationale -- The MA Strikes or Sweeps skill is the basics, the general martial arts knowledge. Individual Styles then build upon this in various ways. The basic rationale is that your specialized knowledge cannot be higher than your basic knowledge, and yes, this is something that carried over from HARP's RM roots. It works as a balancing factor, to keep characters from dumping too much into a single skill, and then becoming overly powerful.