It is a great idea and I will use it. Most other collectibles are traded on the basis of documentary evidence and not locked in plastic. Comics were once the same. Aside from foregoing the environmental hazard, sometimes I want a second opinion on my books and also want to keep it raw.
This would also fill a void at shows, e.g. where two people want to settle on a grade to facilitate an in person sale.
I have seen boardies sell books here and elsewhere, de-slabbed, with support of a CGC label. It worked just fine.