IMO, the step platform isn't necessary unless:
1) your weight plates hit the floor before you get to your safe ROM at the bottom (as mentioned above)
2) you are flexible and strong enough to go deeper WHILE KEEPING A STRAIGHT (as in 'natural lordotic curve') BACk and NEVER ROUNDING. (Think of yourself as one of those bird toys: the ones that dip their beaks into a glass of water and bob back up--or am I dating myself? Anyone remember those? The upper body stays in the same relative position throughout the move--no curves--and the only movement is from the hips.
As to the 'lift from the bottom' question.
True, traditional deadlifts (I'm surprised Cathe didn't put some in STS--they are SO effective, and as much a total-body move as the squat. We could have done them using our step risers to hold the bar a bit higher, which is something I've done at home) start at the bottom, but the movement pattern is different from Romanian DLS.
I can't see safely picking up the bar from the floor in RDL position (you have to bend your knees to pick it up) and starting the move from the downward position. After the bar is in place, it never touches the floor again--unless you have extreme flexibilty.
If you were to elevate the bar somehow, so it starts at what would be the bottom of your safe ROM, then yes, you could do it as a 'starting from the bottom' move.