" I would love to be able to buy a few of the high end features, partitioning, mirroring, compression, on a per instance basis."

That's not per usage, that's more granular per-feature.

IMO if you really paid per-use, the overall price would be higher because someone would have to administer it and bill you. Individual credit card transaction with admin overhead including occassional dispute resolution costs about $8, electronic invoices are about $12.

Also, what would be per-usage fee? Say you would pay a certain amount for each boot of Windows or start of SQLS. (You already get 50 "free" boots before you have to enter license.) What if you had to reboot because it crashed?