IMO I don't think it's a stupid idea. I haven't used them but think the H/T is probably a good choice for sand as it is a road tyre, so will not dig you in as much as an A/T would.
So long as the tyres are in good condition (undamaged) then I would prefer to take tyres with some use on them as any manufacturing defects should have shown up (four of my tyres were made in 1989, 2 in 1994 - maybe this is a little extreme!)
Spend some of the money you saved on a 2nd spare wheel/tyre instead :-)
Hope this helps,