Just some insights on this view, using mostly SMT you likely don't need a PCB holder for
assembly, particular without any components that keep the PCB from lying flat, which is preferred in SMT components. If you plan assembly to leave tall items to the end, big items like transformers or electrolytics can actually be the platform, balanced against say a prop piece of wood or the dreaded helping hands!
Free_electron mentioned using 'silly putty' as a prop, and that has an added value of being sticky without leaving a residue.
However, in troubleshooting, modding, hacking, removing or salvaging parts, a PCB holder of some kind is very helpful, particularly with through hole parts. Through hole also means making jumper connections, removing or adding headers, etc., A question then becomes which design offers the most utility, at lowest cost. I like Panavise because its been around forever, and you can get parts anywhere including used at eBay or swap meets for far cheaper than new.
So in the end, PCB holders I think aren't "useless", as the word implies of no value, but more "used-less" than you expect, given smaller PCB sizes and lower parts count and SMT components.
I find however, that most of what I do is modify rather than build from the ground up. When I build, its dead bug style and a PCB holder is very helpful when it comes time to etch out the copper with a dremel tool to hold the PCB from dancing about, and keeping both hands free for the fine work.
A simple test is try to assemble without holder. If you find yourself using a hand to keep positioning the board in any angle but flat, or holding it down, you probably can use a holder. The cheapest one that can work is the controversial helping hand, and keeps you from spending too much for something that may be 'used' less than you expected. If you find you keep fidgeting the helping hand while you're doing the work, you either don't have the best position possible for it, or you need something better.
.......
these 'pcb holders' are useless.
While his tone may not be exactly avuncular, his point is valid. Those things *are* pretty much useless...