Except you said give everyone equal experience after they join.
This means Marcus is getting his share of the kills in all of those levels that he is a complete experience drain. Sure, that'll make him a logically higher level because by the time everyone is hitting 20 he'll be a whopping 20/4 or so, but that is completely against how the game is actually played. You don't use the higher level characters early because there is absolutely no point to it, which basically nullifies their higher level. By that same token, you use the lower level people (if you are choosing to use them) a lot when you get them because they get such awesome amounts of experience that it becomes more efficient to do so.
Again, I can't speak for 9 or 10, but in FE7 and 8 I can't see the logic behind rewarding characters who better be on your bench for most of the time you have them. Unless you plan on saying something like "Marcus starts getting real experience at mission 18. So instead of saying he joins at mission 11, I'll say he joins at mission 18 and start him from there."