Personally, I believe that the cost of the segments will be less, overall, then the cost of HSS knives, and far less than carbide knives. There are advantages to the HSS--a better edge being the major one, added to lower start-up cost. The carbide knives can be sharpened, but not in most hobby shops (also a problem with any jointer knives over 6" long). The use of old knives with old wood is a good idea, but think of the extra set-up time needed. And what happens if you only have one set of old knives, and hit a hardened nail or two right off the bat.
I'm stuck with HSS knives for the foreseeable future, but somewhere down the road, I'd surely love to have an 8 or 12" spiral head jointer. And about 80 extra segments.
Charlie Self "Political language... is designed to make lies sound truthful and murder respectable, and to give an appearance of solidity to pure wind." George Orwell