I personally don't feel that it is necessary. If you are talking about using it only for decaling, I really do not think it is a must. For decals, most of the time all you need is a glossy or even semi-glossy surface so you avoid silvering. I use MicroScale's MicroGloss for this and it works well. Depending on how you appply it, it dries faster than Future, too. As for the drying time, it doesn't take 24-48 hours to dry as a law. If you put a thick coat of Future on then it will take longer to dry, or if you have a few spots on the surface with deep grooves (like zimmerit) then it could take longer. In my experience, and the way I use it, it only takes 30 minutes or so to dry. I apply it only to the area that will be covered by the decal, in a thin, brushed-on coat. If I am not satisfied with the level of gloss, then I may add a second coat after the first is dry, but one is normally enough. If I am in a real hurry, then I whip out the hairdryer and speed things up with a little heat.
In a nutshell, the Future isn't as vital as the smooth or glossy coat underneath the decal, regardless of the medium used to produce that coat. Hope this helps you out.