Explain how Regal helps out Barrett?
A manager helps get a guy over and covers his flaws. Regal can't get in the ring and wrestle for Barrett, so the one thing Regal does better than Barrett, he can't do as a manager. Barrett is over, adding Regal won't make him anymore over. If you say "well he could mentor him and make him better in the ring". Well, regal can do that without being on screen. Regal can do a lot more good for the company staying backstage as an agent and helping more guys.
Paul Bearer was a voice for Kane. They needed someone there to explain why all this crazy shit was happening and to help tie it all in with the Undertaker. Kane couldn't talk.
Your logic is decent enough, but you've applied it to the wrong situations. What you're saying is like saying having Usain Bolt drive a Ferrari will make it go faster just because he's fast. Barrett already excels at all the areas that a manager helps with, so adding one does nothing.
Regal has an excellent sinister heel personality. He can draw heat, and in turn, by association, Barrett will draw even more heat. Is Barrett the number 1 heel in the company? No. Does he want to be? Yes. Tweeking his character would only aid him and give him even more credibility. By all means, leave Barrett as he is, but having Regal as manager would do him no harm whatsoever. Lets remember, Barrett hasn't even held the World Title yet. Sheamus, Swagger (poor run but faith in him), all held the gold within their first 18 months so dont tell me there isnt room for improvement.
WWE is so thin on the ground with main event talent right now and he still isnt there. They put the belt on veteran Mark Henry to carry the major heel over on Smackdown.
PS - With regards to Kane, I was talking about 2010 when Bearer was Kanes Manager!!!!