Ah yes, a TB spacer is good for that kind of thing so you don't have to drill holes in your throttle body. Good point. Snazzy plugs alone are useless "upgrades" and just a waste of money. However, there are instances after doing a number of upgrades that you'll need a different plug type (usually colder or hotter range plugs depending on application, N2O usually uses 1 step colder plug for example).
It's kind of that way with all things though. Most of the small cheap bits can be thought of as support for the big bits.
You have a displacement, and within that displacement you can either try to burn more fuel, raise the rev ability, etc. All the small things (intake, exhaust, etc) can only make a small difference unless your engine is completely unbalanced (e.g. "detuned" and with a woefully small intake, restrictive exhaust, etc). Once you start with the big stuff (e.g. increasing displacement, adding boost, etc), then you have to do the small stuff because the small stuff your engine came with are now insufficient for what you have.
Calling them non-beneficial is not wrong exactly, but they are *required* to be changed once you start changing a lot of the bigger, more important bits.
Different plugs on an otherwise stock engine are probably pointless, but once you start adding boost, etc, you will likely have to change them because the oem ones are no longer correct for your application, and at that point they are required.