Random maps just need to be chosen well; if you can get across the map in one turn then they're probably too simple. You definitely want to have a number of peaks and valleys so that mobility is reduced. On very complex maps, usually you can walk almost anywhere with ease, random maps help add variation to this, and make rope use a bit more strategic.
I definitely think you need to play well on both to be a complete player, on randoms you just need to think about placement a lot more. With fewer places to hide, you need to balance a number of things: having the best locations on the map (the spots that seem like they'll withstand BnG the best, as well as offer good hides to BnG from), having access to the best high ground for SD, having enough close worms to reinforce yourself, having some open worms to attack with (or possibly a worm for skunking), and especially having enough worms hidden away so you don't risk getting pummeled immediately.
On random maps, you really have to formulate a plan of attack, and this is almost entirely absent from many complex maps, where it's always at least an option to hide away and BnG without ever attempting to reach your opponent. There are many more instances where you can stop thinking during complex map games, but not so much for randoms.