Small bikes are both cheaper and I think it can be more fun to go fast on a small bike than on a big bike...PLUS you end up in less trouble when things go wrong anyway.
I might ramble for a minute.
The motard track days work for me because of that small bike thing. Kart tracks are all about corners and there's not much need for straight line speed. And you're right, when you crash, you're doing it a lot slower. Usually you pick the bike up and keep going. No smashed fairings, bent frames and the like. You might scuff axle and bar end sliders. Or break your collarbone, if you're me. Plus, the crew are a lot of fun to be around. Less attitude for sure. Compared with track days at a full size track, you show up there on a small bike, there's always some element (usually in the green/slow group) with a point to prove. Straight line heros. Don't get me wrong, most people at full size track days are lovely people. I've taken my big boy's bike (TRX850) to the full size track and it's fun but i'm not excited by the VMAX thing. The Enfield's good there too, in the corners. Straights are too long though.
At the kart track you can run what you brung and still have fun and fit in. My DRZ400SM is just the right amount of bike for me.
Anyway, 10 out of 10 would recommend doing a track day, just to see what it's like. Have at it if you can.