It really depends on what cartridge your mounting and if your trying to add extra mass with the soundsmith's. I needed a few extra grams to get my resonate freq right so I used their brass screws and they did the trick.
Also since my cartridge also needed a nut on the bottom side, aka they dont screw right into the cart, those screws prevent you from having to have fumble around with a little screw driver while balancing the nut, which is nice.
Absolutely correct, every time you change one parameter it effects everything else. I set VTF after all other adjustments are made ie the cart is aligned with a MintLP. I only adjust the cart screws to get the overhang and align the cantilever with the protractor and then tighten the cart screws and I am done. Last of all set the VTF. If you using shims in the head shell to adjust VTA the cart screws might come into play, OTW, adjusting cart screws has IMO little to do with VTF or even VTA.
Agree these are useful to add some mass at the headshell.
Once I have a cart properly aligned and dialed in, the range of VTF tweaking it needs (and I tweak VTF almost daily) is not enough to make realignment necessary. My VTF tweaks are rarely more than a few thousandths of a gram.
YMMV, as Sunnyboy said!