What is a sense coil? Not all 8 wire steppers are unipolar!

I am working with some 8 wire stepper motors that have a very strange
characteristic. I had done lots of research before purchasing, and
everywhere I read that an eight-wire stepper motor would always be
universal. I found that two coils have a high resistance while the
other two have a low resistance. This was really confusing until I
got the manufactures data sheet. It turns out that they are bipolar
motors, with two sense coils. From what I gather, you can use the
sense coils to determine if the stepper has actually moved to where
you wanted it to go. Has anyone ever heard of this before? Is this
correct? Seems like an expensive way to implement an encoder! What
type of product would use such a feature?

I have successfully driven them in bipolar mode (297/298 chopper),
ignoring the sense coils, and that works as expected. I have many of
these motors and I am trying to design the cheapest/best driver for
them. I would like to include the sense coil feature if it is useful.

The bipolar drive coils are 1.25 ohm rated at 2.1 volts 1.65 amps.
My applications for them always include driving at least three
motors, so I think I will do the logic in a $5 PIC versus $24 for 3
297s. I am having trouble selecting the drive circuit. The
complication comes in when I try to get pricing information. I am
looking at the l2998 from allegro Microsystems, which includes
internal diodes, but can't find a supplier/price. I am interested to
hear others opinions on the easiest/ cheapest drive circuitry. I am
also interested in references to learn about h bridge design if that
would be cheaper for this current level.

It seems that drivers are limited by the amount of current they can
provide, which makes me wonder why the coil wouldn't be higher volts,
lower amps to accommodate. The 5804 is such an easy solution, wish
they were unipolar

Thanks for any input.