My point, and question really because I don't know, how does 5 years as an MEI compare to 5 years at a regional for getting hired by a "major"?? I proffer that 5 years of 121 time is more valuable for your resume
In today's market I would say you have about a 0% chance of getting hired at a major without prior 121 experience. Even with a large amount of 121 time your chances are still slim without internal references.
Now, that being said, I think that the 300 hour FO isn't really a great idea. You can teach a guy procedures, but you can't teach experience. When I say experience I am talking about things such as judgement and the ability to think outside the box when the time calls for it. These are things that sitting in the right seat as a CFI will teach you. I truly do believe that teaching will make you a better, more well rounded pilot. Obviously these guys that go through programs such as MAPD make it in their careers, but I think that it is unfair to ask a captain to help bridge the experience gaps with these guys while you are flying around paying passengers.
Just my two cents on the matter....
