The problem I see with flexraid is not just that is being commercial, that is OK for me in principle, but I think the model the author has adopted is problematic: VERY expensive IMHO (100$) and no way to really try it before buying, so you have to have faith in it and expend a hundred bucks... that for a product with an insignificant user base means very few people will use it, and so there is a great risk that the author will not make any meaningful amount of money and so a high risk of abandoning it, and that leaves users (now purchasers) on quick sand... a pity, since the product has great potential and the interface is really slick (although a bit confusing, but that could be improved with time).
OTOH, I do not think that what the program does right now is so hard to do (although the author has some really ambitious ideas for the future); it does two things, 1) storage pool (combining some file system in to one) that already do a number of free products on linux (unionFS, there is at least 3 free open source implementations that I know of) 2) snapshot RAID, that is calculating parity at file level so you can rebuild any missing data up to the parity information you keep. That is exactly what some opensource utilities of the "
parchive" family do, like QuickPAR (windows) or PAR2cmdline (linux, windows, mac). So, to implement functionality analogue to what FlexRAID does you do not need some high level math understanding, simply some way of organizing the original files and then use the algorithms/functions from the PAR2 source code to compute the parity or reconstructing the original files from parity. Of course that is not at all "easy" and it is a lot of work, but it is not rocket science either since you are basically incorporating other people work, that exactly is what open source is for.
The advantages of an "offline parity" drive (or more) and unionFS are enormous for the kind of storage that most users will need for a media server for a few XBMC installations on a single home, so flexraid or other similar utilities are a great companion to XBMC users.