Stuff like that is really common. Biaggi, Stoner, Checa...all sorts of world caliber riders perform testing duties (when they cant get a ride or are retired). And vice versa, the test riders are usually fast enough to fill in for races if the main rider is injured.
A test rider is useless if he cant push the bike to its limit, while feeling what is going on (how the changes affect the bike) and being able to give good feedback.
Some teams have multiple test riders. One of which is a lesser rider, somebody that can give very basic/generic feedback on geometry, tire wear, comment on HP and braking, etc. Somebody that can do alot of the "grunt work", stuff that maybe some of us could do. Then they have a 2nd test rider like the aforementioned world caliber riders to really push the envelope and get the bike as close to race ready as possible, before the primary rider (racer) gets on it.