So I gather that Dexter is a brand name for torsion axles. 

Are these axles somehow more desireable than other makes and if so, why? 

Has the technology for torsion axles changed in the last 10-15 years, or has it pretty much remained the same?  - i.e. would an older trailer with torsion axles have a comparable suspension to a newer trailer?  Thanks!

I think Dexter is the dominate mfg in the torsion axle business.  I see some other brands when I searched for replacement axles.  the cheapest replacement I found was a place in Salt Lake that said to bring in my old bent axle and they would build me a new axle to be a direct replacement.  Whether the parts they use were from Dexter or 3rd party parts to build those kind of axles are readily available I don't know.

I think the technology is the same. Maybe they have improved on the rubber  or other componets.