Yeah, I'm guessing those suit commuters aren't riding too far or very fast. If I was riding 2 miles at 5 miles an hour, I wouldn't sweat too much either.
For me, a shower before I leave helps my sweat not be stinky when I get to work (It's "clean" sweat!)
Also, My commute to work takes 40 minutes because I'm trying to go slow and minimize sweat. The ride home takes 30 minutes because I don't care so much about sweating.
However, to answer you other question about rain gear...the simple answer is that there is no magic material that stops rain and breaths. if it breaths, it's because there are vents and openings in the jacket. If you want to try a jacket with wind-proof front and vented back, that's probably your best bet in light to moderate rain...In a deluge, you're going to get wet, and if you wear a jacket, you'll likely sweat as well.
Since (in my area of the world) it is mid-spring, and summer is approaching, we are fast coming to the time in which I will wear the same shorts and short-sleeved synthetic shirt whether it's going to rain or not. If I get, wet, it won't matter, since I'll be changing anyway.