So what will a TW, SME, Basis, Graham, etc, give me that a Jelco or Audiomods won't?Maybe a lot, or maybe nothing. Your question argues your fundamental understanding of high end audio and audiophiles and sounds more than a little judgmental. Not everyone wants, needs or can afford a "high end" (aka expensive in your parlance) tonearm. Those who do or can may be looking for features or adjustability that are not available in less expensive tonearms.
Cartridge/tonearm/turntable matching is a complex art. Not every tonearm works best (or at all) with every cartridge and every turntable. Got a Lenco L75? If so you are probably not going to try and mount a Thomas Schick tonearm on it.
Let's say that you want to run an Ortofon SPU cartridge. You can do that on a Jelco (it will work but it's not the best choice, IMO). But the Audiomods?
No. I'll use another example: I enjoy the London Decca Super Gold cartridge. The best cartridge/tonearm combination I have heard is with the Fidelity Research FR-66. A great tonearm. Is it worth $3,500+ for a 25+ year old arm? Not to me, but every one that comes available gets snapped up pretty damned quickly.
Use what works for you and leave us to do the same. Please.