Lookout is required by law and a bloody good idea. Doesn't matter if for weather or traffic. There are quite a few weather phenomena and effects that cannot be seen on radar, would be nice to see it beforehand instead of sleeping in a shaded flightdeck until you get a nasty surprise. Same for TCAS, of course you fly the RA, however it is quite often not even necessary to run into an RA and even more important, there are still planes around without Mode C/S which won't picked up by tcas and don't forget, it is quite legal to fly around for 10 days without any TCAS at all.