If any tyre is left exposed to the weather over time they can harden and will no longer be safely usable. With semi's, you lose the benefit of the high grip levels. I will check the date when I get a chance to go to the workshop next week, but I know they are still good so i don't mind either way, if they don't sell I will be keeping them to use myself.
Matt - they wouldn't last long if you do a lot of street driving on them, but if you want plenty grip and only drive it one or 2 days a week then could be an option. Of course you have to accept the tyre noise and stones flicking up, tram lining etc.
I'm driving my daily around on RE55's but that's because i'm too lazy to change wheels before and after a track day and i don't drive much at all anyway, only on weekends mainly.