(EDIT: I guess this is at least partially about whether to let go of the darned thing. I'm having trouble with the idea of not having a bike, and it's a lovely bike. But it's also just sitting and depreciating most of the time.)
The registration on my Aprilia's about to lapse. Oregon requires insurance to renew. My bike isn't insured, as I don't ride it on the street. Insurance costs more than a track day, and that's where the fun is.
In the short term, I really don't care that it's not registered. But when it comes time to sell it, current registration would seem to make life easier, and there's a financial penalty (and I actually have to go to the DMV, I think) for expiring and starting over.
I guess the most accurate reflection of usage is just to let it expire, not worry about the minor repercussions of that, and either go through re-registration if I ever decide to ride it on the street, or let the buyer worry about it.
Or maybe I should look harder at selling it, since it's looking more like I'm not going to make it to any track days this year, and revisit buying a track-prepped bike if my schedule improves 
The Aprilia is a bit nice for a "track rat"...
What do I do?
If you don't have plans to ride on the street anymore, selling it and then picking up a dedicated track bike for less is the smart thing to do. But for most of us bikes aren't about the smart thing to do and you will probably never find another bike that stirs your soul like the Ape...so I'm absolutely no help. 
In reply to secretariata:
A nice summary of the situation, though!
I think I could be pretty happy with a different track bike; part of it, I think, is a concern that if I sell the Aprilia now, will I get around to picking up another track bike later? I guess maybe either my schedule will straighten itself out and it will be Time For A Track Bike, or if it doesn't, I'll just be wasting a lovely Aprilia in the meantime... 
Sell it now, pick up a track bike this winter when such things are cheaper.
Do you have the option of non-op registration?
On a side note I think it's really dumb that we can't drop insurance on a vehicle and keep the registration. I tried that when I left town for 4 months and was told of the horror that would await me upon return.
How about getting an insurance policy that covers when the bike is not being used? I have insurance on my bikes through Farmer's Insurance and it's really pretty cheap for on the road coverage (I don't have the numbers handy, but it's like $70/year for basic liability) and I think they and probably others offer even lower fees for bikes that are stored.
