All good points PC9. From what I have seen in U.S. military primary training (USN and USAF) they end up doing a number of reflys, warmups, etc when there hasn't been a sylabus flight within the last week or so. In some parts of the world it may always be sunny and warmer but that is not always the case in USofA. It is also a factor to have an IP in the other seat that has a real safety as well as psychological effect.
The T-38 is an altogether different and supersonic kettle of fish, so I was thinking the primay training vehicles (T-37s and the turboprops) and light jets with reasonably benign handling qualities (Hawk, some of the Aeromachi products and I'm tempted to say Alphajet but that is getting a little more complex and immediately hairy than the others. Being unfamiliar with the L-39 maybe I shouldn't have included it.
All the above said I would be very leary of a high performacne aircraft flown by anyone who did not do it fairly constantly that did not have a significant support structure of instructors, training and possibly simulator (even just a procedures trainer). And there is a big difference between being legal (3 T/Os 3 Landings in category class and type every 90 days) and being really proficient. I'd take proficient any day of the week.