Another reason not to carry a gennaker is that it's virtually impossible to pull off a foiling jibe with one.

The gennaker has to be partially furled, jibed, then unfurled. The extra speed/time gained when you're up on the foils is more than offset by the time spent off the foils in the jibe.

At least one team (ETNZ) has a self-tacking jib - which may partially explain why they were the first to achieve a fully-foiling jibe.