goes back to wet vs dry weather. dry weather my rim brakes are more than fine, in wet weather its not just braking but tire grip, so you'll pull back in either case.
On most surfaces, I have no problem lifting the rear wheel with my front brake even in wet weather; tire grip isn't a problem in a straight line. Cornering adhesion is trickier, so you can't go as fast in wet turns. But that means more braking, and more braking throws the the advantage to discs even more.
Minor weight penalty? 600g on average...
Currently, discs have a significant weight penalty, but let's compare apples to apples. Someone upthread suggested that the higher-end weight penalty is more like 400 grams. Plus, disc-brake rims tend to be a little lighter, though that might already be incorporated into the 400-gram number.
There's a weight penalty, yes, but it's not as big as you suggest. More importantly, that penalty will only shrink in the future.
In theory, there's a weight penalty to using discs instead of rim brakes on a mountain bike, too. But as soon as it became hard to find race-level XC forks and frames that accept rim brakes, consumers' concerns about weight penalties evaporated. The lightest "raceable" mountain bike I've seen is the 7.2 kg Open that Fairwheel built--with XTR discs. Surely there are some lighter, raceable hardtail mountain bikes out there, but do any of them have rim brakes?
The lightest mountain bikes now have discs, not rim brakes, mostly because the lightest mountain bike parts are made for disc-braked machines. At some point, development of rim-braked road bikes will cease and disc-braked bikes will be lighter than current rim-braked bikes. This is a question of when, not if.
That said, anyone who prefers rim brakes should ride them. I don't, which is why I'm saving my pennies for a disc-braked Madone. I'll ride what I like and you ride what you like: everyone wins.