The article mentions it already has a two-stage design, shifting bits and then bytes, so it can't be about shifting more than one bit at a time. Anyone know why?
If you're using MOS pass transistors for each stage, you lose some voltage at each stage, which limits the number of stages. I think this is why the 8087 (and the 386) used two-stage shifters rather than logarithmic shifters. I don't know how the circuit area compares between the two approaches--two more complex stages vs six simpler stages--but I suspect the two-stage approach wins.
I sometimes wonder if some design decisions were made on that basis.
You might think that the 8087's shifter would be a regular grid, easy to lay out by hand. It turns out to be very optimized and irregular. (I traced it out by hand and it was a pain.)