Well, right now it moves in ten seconds (or did in earlier games) on my machine. How many ply was that? Maybe two (its move and my move)? So four ply would be about, say, a minute or two? Not more than five I suspect. With a short wait time you just doubled the ability of the AI to play. Sure, the cost increases fast, but you can make some improvements if the user is simply prepared to wait.
Add in the ability to minimise to a system tray icon while the AI thinks, and you're onto a winner
No wasting processor cyles on tree animations and scrolling, etc.
The way the series is going each game is built on top of the last anyway, so I suspect the algorithms for the AI will get better, and not as an additional cost since it will be a new game. All the while you're teaching users to give the AI time to play, and thus increasing its smarts cost free.
Plus, processors WILL get faster. No point thinking so short term that you have a fixed time setting as two years from now that time will be a joke.
I wouldn't have thought turning the "think time" setting into a variable would take much work. No harm in adding it as an "is this worth it?" feature.
By todays standards, my P4 3Ghz processor is stone age anyway. The lowest end Athlon available on most hardware sites would equal it at the very least. No OEM ships processors slower than this any more.