At present sys-power/nut supports 51 drivers, all of which are enabled by default. This means either you build a lot of drivers you don't need, or you have a huge long list of drivers to turn off in /etc/portage/package.use. It's messy and awkward, and realistically, nobody is ever going to need ALL of those drivers except for testing purposes. Probably 99% of the time, at most two or three drivers will ever be needed in a given installation.
(Real-world example: I've just installed it after replacing an APC core UPS with a Cyberpower; I experimented with usb_driver_snmp-ups, ups_driver_powerpanel, and ups_driver_usbhid-ups, and after testing the ONLY one I actually need is ups_driver_usbhis-ups.)
Change all of the drivers to default-disable, and add a ups_drivers_all flag to turn them all on just for the vanishingly rare case when yes, somebody REALLY DOES want ALL the drivers. Then people can turn on just the few drivers they need instead of needing to turn everything else off in order not to build fifty drivers they don't need.
I would add my voice to this problem. Came here in search of easy way to build just the drivers (really, one driver) needed.
I think this should probably be closed in favor of bug 598619, which has the more Gentoo-appropriate proposal of using USE_EXPAND variables.