Originally Posted by
The_Cretin
There is a great disjoint between power and PE inside. In other words, it hurts a lot more to produce the same amount of power and you often can't sustain it for as long. It is better than doing nothing though.
I think you're exaggerating the difference between indoor and outdoor. In my case it's less than 10% and I can get my indoor power up fairly quickly in a couple of weeks training 3-4 times/week.
If someone is losing fitness while training indoors it's more likely due to a fewer hours/wk rather than some intrinsic trait of the indoor trainer.