my wr sits outside in front of my uni all day. as you'd expect with german weather, it's pretty changeable, and the bike can sit in the sun for 8 hours one day, and in the rain for another 8 hours the next day. bike won't mind at all.
when mine was new I always tried to keep it clean and keep it away from the rain... but that only lasted a couple months. in the meantime I convinced myself the bike looks better dirty
the only thing that bothers me with a wet bike, is sitting on the wet seat. even with my rainproof gear on, it's kinda nasty sitting on a cold wet seat that's been sitting outside under the rain for half a day. I live with it as my commute is short, but for me, that would be the only reason to consider putting a cover on it. to have a dry seat