The problem with relying on a raid healer to keep inspiration on the tank comes from that the heal on him or her also has to crit. You may throw a heal that land on the tank every now and then but with current levels of crit on gear (and often even more so on priests not geared for tank healing) you have to be a bit lucky still to land a crit on the tank every 15 seconds.
Btw, didn't Zusterke had a guest post at World of Snarkcraft about inspiration uptime with some math attached once upon a time? If you can find that it may have some relevant info still. It's most certainly for pre-cataclysm content though the inspiration mechanic hasn't really changed since it went from increasing armor to damage reduction late 2009.