For all intensive purposes, the slider is basically the same as the Transformer 1.
IMO, the reason it is not more popular are these:
Heavier than just the tablet
less battery life than the tablet only Transformer
fixed screen angle
hard to find a case for it since it is a unique design
With my TF101, I get 15-16 hours on a charge. I know the slider has maybe 6 hours if I recall. I can go with or without the dock.
What is the current version of firmware on the SL101?