Hi Moriaty
We have gone down this line, pretty much for the reasons you outlined. Unfortunately one can't crystal ball gaze too much when it comes to Microsoft, Google et al but I think that even if they change the scheme they would have something comparable as they seem keen to embed their software in schools and the cheaper they make it the more likely this is going to happen.
IMHO I would go with it but you could always ensure that it is understood from the outset that this is based on the current scheme that could be subject to change.
Regards
Kury