I find nothing illegitimate about practicing with a computer to learn the game more strongly. Only with using one to make your moves for you in games.

As for Cake SS, it's ok. I'd rank it fourth among programs, behind WCC, Kingsrow (best free one) and Nemesis, the real best. My reason Cake isn't ranked top is its opening play has weaknesses, especially in the weaker 3-move openings and in some GAYP openings like the Souter, while Kingsrow really doesn't have many opening book weaknesses, and for the few it does contain, it never plays those lines. I've only beaten it in 3 drawable lines of play while I've beaten Cake SS on about 5.