In the past, vvt for bikes added a lot more complexity and potential problems for relatively little gain.
http://www.sportbikes.net/forums/showthread.php?t=356133
One of the good responses, from Isaac:
VTEC doesn't add more power, but it does make engines have broader powerbands, thereby making the car they're attached to faster.
Like ithicaplasma said, it's by using a lower lift, lower duration street type cam lobe profile down low, which improves low rpm gas velocity and thus improves low rpm torque. Then when you hit the changeover point, the VTEC system locks out a valve system that causes the high lift, long duration cam lobes to actuate the valves, improving overall breathing in upper rpm due to increased max flow into the cylinder.
If you used just one of those profiles or the other you'd lose maybe 2,000rpm of the powerband in the process, either rendering the engine gutless down low until it came "on cam" with the aggressive cam lobes, or making it run out of breathing room up high when the milder lobes weren't opening the valves enough to support any more airflow.
The reason it's never been used in bikes is twofold:
1) Since VTEC and various other camshaft phase rotation type variable valve timing systems like BMW's infinitely variable VANOS are designed to broaden powerbands, and to a lesser extent control fuel economy and emissions, they're much more helpful in heavy, relatively weakly tuned cars. The average car without variable valve timing is lucky to have 4,000rpm of torque band to play with.
Bikes already have powerbands that in some cases extend 80% of their peak torque more than 10,000rpm wide. That's plenty good enough, especially when any deficiency in torque down low is rarely missed due to the bike weighing so little.
2) Those systems can be heavy and complicated - nearly as heavy as a car system, which makes them much more noticeable on a bike as a percentage of weight gain. As I've already said, bikes don't really need it, so they're not worth the effort, not to mention they'd tack on another grand or so to the bike for a marginal gain in acceleration.