i don't agree part of making a sunny day team is to deal with that kind of situation take rhydon for example hes naturally resistant to fire because hes rock but he is also weak to water the sun reduces the power of water moves reducing his weaknesses so he actually is a good pick for a sunny day team as its helps neutralize advantages an opponent gets from the sun as well as covers your own weaknesses like flying.
other good things include things with thickfat like snorlax and haryima
thick fat resists ice and fire
which are both weaknesses for chlorophyll users like exeggcutor, victreebell, shiftree pardon my spelling
yes your team should be able to function out side the sun as well as in but you want to pull as much advantage as possible out while in the sun
if that means u run gigadrain over solarbeam on exeggcutor so be it. you still get a strong hp fire and patch up the speed.
i'm not saying sunny day is easy in fact it takes more thought than just a standard team. its a way to bolster your offences in a meta dominated by walls.