I guess it depends how much those extra rewards are worth to you. I was running my own numbers and if I carried around every card I have and always used the card with the best options, I'd earn at least an extra $100 annually. But I don't do that because it is a pain to carry around so many cards and keep track of which to use when. I use the card that gives me the best rewards overall (which is still pretty awesome since I calculated which one that was based on my specific spending). A $50 net gain sounds pretty good in my book. Depending on your spending patterns it could be worth much more to you.
The only downside I see is redemption options. If you have very low spend on a particular card, even thought the rewards may be higher it may take so long to reach whatever minimum redemption level the card has (or a sane redemption level that value points less than one cent each) that you may be better off getting slightly lower rewards on a card you use more often so you can redeem sooner. In that case just don't add that card to Wallaby.
It might end up sucking if they don't have good algorithms. (There are a lot of variables, not the least of which are those rotating 5% cards.) Only time will tell. It's certainly an interesting idea.