think about it this way, MHZ. In order to use Wally the way that you are proposing, you need to get the Pokemon that you need (in your example, Marill) (1 card). Then, you need to get the other guy out of the Active position. Most of the time, you won't want to discard energy in order to do this, so you'd need either Switch or Balloon Berry (2). Then, you'd need Wally to get the Evo that you need (3). And, even after all of that, you still don't have a S2 out, only a S1.
In the RC and PETM example that I proposed above, you still need 3 cards, a Basic, an RC, and a PETM. However, unlike Wally, you don't need to have the guy active in order to use the combo. Also, once you're done with all of the cards, you're likely to have a S2 on your hands, or a S1 with the same amount of effort. Wally, the way that it is currently worded, is basically the same for speed as RC/PETM because it will need a Switch in order to do its dirty work. Once the E-sets rotate out, however, Wally will become extremely powerful, now just is not his time.