How about turning the question around? Why go with the CF, especially with all those negatives you mentioned? Have you heard of any boats breaking in half because they used glass instead of CF?   

The Mill Creeks and Sasafrasses have cockpit openings every bit as large as the WD doubles and have sold for years without needing extra reinforcement, either.

It's your boat so do what you want, but the biggest negative in my opinion is the waste of time, money and effort for something you don't need.

Laszlo

I built a WD Double for my daughter and her family, they normally have two adults and one kid in the middle and it's just glass. Never had a problem with strength. You might also want to consider that's a lot of blackness on the bottom in a wide open cockpit that's going to absorb a lot of heat. In Florida (where we live) that would make for one hot cockpit. 

George K

   A different way to get a strong but light boat is to use 4oz glass inside, with an additional layer on the bottom panels only, to just above the fillet. No tape in the cockpit keel or chine joints.

So I cut a piece that fits on the bottom, and another that will drape all the way from rail to rail, and fold them out of the way. I also cut a piece of peel ply that will fit from chine to chine, fold it and set it out of the way.

Add filets to the tack welded seams (wires removed), then gently drop in the smaller piece of glass and dry brush it into place, gently pressing it into the filets with a dry chip brush. Gently. Then add the full sized piece of glass and smooth it out somewhat. Pour resin into the bottom and wet all the glass, avoiding bridging and being careful not to mess with the fillets too much.

Then add the peel ply over the bottom panel, squeegeeing it firmly into place, adding a little resin on top of it to ensure it is full wetted.

The peel ply leaves a very fine woven texture, so no fill coats are needed- just scuff and paint (later). It can also be left in the cockpit until very late in the build, acting as a sole protection layer against scuffs and drips. This alone is golden and worth the extra 20 minutes it takes to put it in. The downside: two layers and peel ply tend to leave a somewhat milky appearance, so varnish inside won't look as good. I like to put VC Performance Epoxy on the sole anyway, which is a two-part, white, very hard finish that resists sand and grit scuffs from dirty feet and remains easy to clean forever.