I guess it's more of a thing for going back and doing it for the sake of doing it I guess. Helping OTHERS who were in your situation maybe.
As for the 4 level gap, I don't understand it too much either. They can stand to make the whole "lol no exp" thing for when you're 10 levels above. That would definitely be enough unless for whatever reason there are several flashpoints between 45 and 50; in which case I'd understand due to being able to grind the lower level one easier if you were say, 48 or 49 at the time.
I'd actually like to see if Bioware has/will address that particular thought.
I went back and did Foundry yesterday to help a guildee out and grabbed the quest before i went in, I was kinda stoked to actually get something out of it when I didn't expect to, It's just dissappointing when I get all excited about being able to do the quest again only for it to "reward" me with 5XP. The bottom line is that the 4 level "you get nothing" gap doesn't make much sense, and from that standpoint, yes we're on the same page.