Plushenko pioneered the 4Toe-3Toe-2Loop and there's a reason why he did that instead of a 4Toe-3Toe-2Toe: tacking a toeloop onto the end is simply repetitive and not worthwhile. Doing a 2Loop, however, looks special. It creates an extra tempo and an additional look for the combination.
Actually, I suspect that the reason he did that was he was training a 4-3-3 combo that he wanted to make history with, 4T+3T+3Lo. He didn't plan 4T+3T+3T because he wanted either to save the 3T repeat for the back of his 3A or as a best case scenario to repeat the 4T and the 3A and not repeat the 3T, not so much because of "looks" of the combination.
Why put the loop at the end instead of as the middle jump? Because it's really really difficult to control the landing of a very difficult jump like a quad with the free leg in front to set up the loop. Much easier to do so from the landing of a 3T, although still very challenging to maintain enough speed and alignment to pull of the loop, especially 3Lo as opposed to 2Lo.
Nobody ever attempted to do a xxxx-3Toe-3Toe (or xxxx-2Toe-2Toe) combination in competition before CoP came about because such a combination is repetitive and doesn't really add anything to the program.
Somebody did attempt a xxx-3Lo-2T, however: http://www.youtube.com/watch?v=xKQRXNj772s
Solves your repetition problem within the same combo, but that's kind of negated by the fact that she tried it twice in the same program (with different xxx). I think the main point was to include a little extra difficulty on an already difficult combo, regardless of aesthetics.
(If only she'd stuck to just the first attempt she likely would have won that event. Or if she'd actually pulled off the second one successfully.)