two things about rain gear:
1. proper rain gear is not just waterproof, but designed for cyclists. among the best (if not the best) is Showers Pass gear (mentioned above). Pick something for your temperature range. You can still overheat, but less so than other jackets/covers not optimized for cycling. If you really overheat, try just slowing down a bit, after all it helps visibility...

2. good jackets increase visibility, which during rain and when sharing the roads with autos is really really important. even in places where rain is more than frequent (hello PNW!) and auto drivers are somewhat more accustomed to it, it's always good to be more visible. in properly downpouring rain i've even been known to use my nighttime flashing lights.

but if you're just out for a summer ride, a little rain is just extra coolant.