The weight training debate seems endless.
The latest quote I've heard from a racer in the Euro-pro ranks (and his coach) when I briefly spotted them in the local mountains was this:
"Weight training is like training for a cross country marathon by walking in the shopping mall, if you're looking to increase your strength on the bike, stay on the bike"
I think the weight debate is endless because there are so many other sports which DO benefit from weight training. And there are others which appear to benefit but those are questionable. In cycling all the studies which support the notion that weight training help aerobic power are either 1) actually rather inconclusive in their findings (or just plain wrong) and/or 2) poorly executed. Whereas there is NO debate that cycling WILL help your cycling (and plenty of studies to prove it).
I personally love lifting, and it is a benefit to my work. That's the only justification I need. Great thing about a power meter is that I am under no misconceptions about what it does for my power. Even with sprinting my power actually took a dive in direct correlation to the amount I was not riding, NOT the amount I was lifting.
I also believe that weight training, abs, plyometrics, core workout, etc dont help on your FTP but if you do MTB racing you sure can benefit from all that kind of training in my opinion.
And by "benefit" I mean arriving sooner at the finish line