I take the points noted above, however, I'm not wearing my best specs in the workshop whilst working but a pair of specs kept specially as working specs. By that I accept that they might become scratched or damaged, which is why I wear them and not my best specs.
If I am doing any serious grinding, either on the bench grinder or with the angle grinder, or any other activity that might throw up rubbish in my direction, like strimming the garden for example, I wear a full face visor, and ear defenders a lot too when it's noisy but that has nothing to do with eyesight protection! When milling or doing lathe work it has been very rare that chips have come my way more than chest high, they are more aimed at my beer store and nether regions, ditto on the bench grinder when grinding lathe tools for example.
So the side shields are an added protection during my normal working "just in case". I cannot comment on how others work and what comes their way, I can only comment on my experience on my working practices.
It is not that £15 is a lot to pay, although it is more than enough as what I actually required was a third of that, or indeed nearly 9 quid for something listed as being under £3, it's not liking having that feeling of being ripped off that I object to. And I don't know lots of other folk to flog unwanted side shields to either, even at a profit!
Chris