The reason of HakeemT's point is that without proper inner support a light steerer tube often comes with a thin wall which is easily ovalised (if not destroyed) by the pressure exerted by the stem's fixing screws.
Hence his suggestion for the use of a T-shaped bung/topcap that runs from the top of the steerer tube all the way down to the expander proper which is then placed in the area where to lower fixing screw of the stem is.
A not so common system that works well provided the steerer is either cut to precise length to accomodate the T-shaped device which also precludes the use of spacers on top of the stem.
If a long enough expander is used and sits inbetween the fixing screws then no damaging compression could occur. Not sure such a device exist. I doubt it.
Another solution and I feel one that works very well is to use carbon mounting paste between the steerer tube and the stem i.e. put on the outside of the steerer tube which allows to reduce torque on the stem bolts by about 30% which in turn should be low enough so the steerer tube is not ovalised or prone to crushing damage.
In all cases work with the utmost care so the steerer is not damaged. Best to use a torque wrench and go easy on the torque you apply. With carbon paste applied to the tube the chance of twisting or lifting the stem clear off the steerer tube is virtulally non-existant for road use.