A scaler is not likely to help SD.
If you have an LCD, DLP or Plasma TV, you have a fixed pixel display and they already have a deinterlacer and scaler.
So the deinterlace/scaler in your receiver would have to be better than your display's solution.
In any case, adding more fabricated pixels to a crappy picture is not going to make it less crappy.
For CRT based TV's, line doublers (which are basically scalers) definitely improved a good signal. They got rid of the obvious gap between scan lines.