"To get a taut pitch you would have to fully tension the stretch cord from the start; thus eliminating any advantage IMHO."
Why? That would only be true if you used very thin shock cord. If you use a small bit of adequately stout bungee, tied into your spectra guy lines, the shelter should be taut long before you exert enough force to stretch out the bungee. The simplest way is to tie in the bungee so that it bridges a loop of your guyline, such that the guyline loop becomes straight when the bungee is maximally stretched. I posted about a variant on that method about two years ago:
I use smaller lengths of bungee now (about 3" total, with 1" showing). The Jacks'R Better cord-in-tube design accomplishes the same thing. I use a bit of bungee in the guylines for the reasons James Marco already mentioned: it protects against potentially destructive snapping in gusty winds. Too much elastic and you get the out-of-control flapping that Roger warns against. Too little elastic (or none, as in standard guylines) and you get high forces on the materials of the shelter when it snaps in gusts. Even the most tautly pitched cuben shelter will snap in strong gusts because cuben shelters almost always have a few wrinkles that give some areas of the material a little slack. About 1/4" to 1/2" of stretch in the guylines, from a stout bit of bungee, spreads out snapping loads but does not allow "flapping to perdition".