What we're dealing with here is a bit of the "big dump" vs. "light parts" argument, which assumes you can only do one or the other, and that all people's output weighs the same. Or that all people will dump at the same time or do the same weight program.
I'll leave that there for now.
What gets completely missed is the psychological aspect of varied training. In my case I usually start to fry mentally well before the wheels come off physically. While cross training, being it lifting weights or cross country skiing, might not produce the exact adaptive gains that you could get on the bike, they can certainly produce quality gains in less trained individuals or at least offer maintenance for highly trained folks. I'm of the opinion that it's very productive and refreshing to associate the bike with something other than pain.
I like weight training and would like to get back to it (haven't done it the last couple of years for a variety of reason). It's different routine, can be done fairly quickly in any weather, provides easy tracking of progress, and lets me work on muscle groups that aid in racing but don't get effectively worked while on the bike.
My peak sprint output tracks in a very linear fashion to my weight training...more weight = higher peak wattage. That's just me. Your mileage may vary.