If by "strange retro" you mean that node-set() function is well known and its usage obvious, that's true at least for me and you. But many times I met people who are doing XSLT and who can't solve some problems because they are unaware of nodeset and result tree fragment differences. This article is targeted exactly for the later group of XSLT developers.
apologies, my comment wasn't meant to be negative, its a good article lifting the mist of confusion re the need for node-sets in a world of RTF. It has got me thinking about nodesets again though....and why we ever had RTF.