There's a problem with your logic OP, you assume people who have gear should have an advantage, or that people PvP in order to obtain gear and said advantage.
Many players enjoy a challenge and gear disparity ruins that, be it in their favour or against. This isn't a complaint against expertise specifically, but it's the largest factor.
Don't get me wrong, I enjoy playing my full BM char in my guild premade, but only when we come across another premade. Stomping ungeared pugs is NOT fun. Most pre-50 games are on equal footing gear and organisation wise, so it's all skill.
Should we have to grind for end game PvP gear? No, this isn't PvE where there's a bigger boss waiting in the next instance. PvP rewards should be aesthetics and other forms of recognition.
If bioware really want to keep the gear sets separate add in a blue entry level 50 set (like the one they removed) with mass expertise, and give BM gear the same amount. That way everyone is on equal footing still.