Desmo isn't all gain's their are some disadvantages, cost and reliability spring (cough) to mind.
The faster the engine is going to rev the stronger the valve spring tension needs to be, as mentioned before about float, it's when the spring isn't strong enough to push the valve back down fast enough before the next stroke, having just consulted Wikipedia I can also add that springs, when of constant thickness and coil spacing have harmonic issues at certain speed ranges, hence the use of multiple or progressive springs on more modern motors to give a damping effect.
Desmo pushes the valves open and pulls them closed again doing away with a spring all together, and I BELIEVE frees up slightly more power at lower RPM where the springs have to be very oversprung in order to function correctly at high speed, however it needs to power the valve into the closed as well as open positions so at certain points will be less efficient, or as the lads on the eng-tips forum put it:
"Conventional valvetrain power requirements are proportional to speed (friction dominant), whereas desmo requirements are purely inertial. So there is a crossover point"
Could never formulate that sentence on my own...http://www.eng-tips.com/viewthread.cfm? ... 426&page=4
Meaning a valve spring is as good if not better than Desmo at high speed, then there is another crossover when you reach the limits of the metal of the spring, to pneumatic.