Does anyone use the inner wheel well "liners" (fenders) on their Spiders?

The plastic ones? Absolutely. My '80 still has original the undercoating behind them, in pretty good condition. I use a large SS fender washer, with oversized SS Screws to hold them in. The original mounting holes tend to get stretched out of shape.

The plastic fender liners came stock after 1979, BUT, that is because those Spiders did not have a metal shield to cover the rear of the fender area. Earlier Spiders have a shield, which when covered with undercoating looks to be part of the body.
Because of this shield if a plastic fender liner is put on an earlier car the lower rear part will not sit flush with the fender arch and may rub on the tire. To make it work the lower few inches of the plastic liner would have to be trimmed.

Good points. I found that these inner shields on my spider had been bent by PO and when I removed the plastic internal kick panels, I could look right out to the front wheel well. I guess my question is, is it worth the trouble to fit to a 75? Not withstanding that it further protects the front area behind the wheel.

If you are missing the original shields than I would definitely put some on. Should make things quieter too.