this is because steady state training will not help with power or sprint type work. this is where Reductimat was bang on with adding strength training to your training routine as it will help your sprint and power capacities which is something that's important for DH riders.
however, spinning classes or a spinning type workout can be effective for improving aerobic and anaerobic performance, it just depends how the workout is setup and of course how hard you work. high intensity intervals if properly executed on a spin bike can make a difference and are effective in terms of time management as you can accomplish a lot in 20 min or less of work.
i was strength training as well doing squats , deadlift , etc. i was working with a trainer as well who helped me . the benefit of spin was improved cadence and some reduced lactic acid burn during some sprints , that being said as soon as the terrain became really physical on the rider that all went out the window .
i now use combined movement when exercising , something that has my heartrate up and down and my body being worked in various movements , for me this has greatly improved my trail riding conditioning and recovery .
i was really able to notice it when racing the Bear Enduro last spring , part of the last stage of the race was on the DH course which i have raced half a dozen times or so over the years . as i was grinding thru one of the flatter more pedally sections i was still feeling strong and was no where near the exhausted shape i was typically in when racing DH years before .