Well, if MTGO didn't require me to buy cards that don't really exist so that I can test my decks out online (effectively causing me to buy the same cards twice for two different playspaces), or to test possible tweaks to those decks, there'd be no need for Apprentice. If it had a free "workshop" section - an area where you could do deck-testing, but couldn't use those decks in online tournaments - then I'd be interested.

Until that time, Apprentice (and Magic Workstation) fill a badly-needed niche. MTGO could easily fill that niche, and do it better, but it doesn't yet.