Not as silly as it sounds.
Many designers don't understand the concept of prepping files for output (Converting fonts and strokes to outlines, Embedding placed art, Color space etc.) Sometimes PDFs contains embedded fonts that will shift to a system default with out a warning pop up or anything.
Sending back a proof of customer supplied art, although an extra step, is just good business.
+1. I had a designer (that I do a good bit of work for) send me a pdf file, and I asked them if they needed a proof, and they said they didn't, the file was ready to produce.
So I import it, and cut it from some material that I purchased by the yard, because to buy the full roll would have just killed the profit in the job (no to mention that it was a goofy color that I would never use again).
I go to install it, and I'm on the top of an extenion ladder taping it up, and my customer comes in, takes one look at it, and tells me that it wasn't the correct font. After going around and around about that is what they sent me, they discover that they didn't convert the fonts to curves.
And because they didn't "approve the file", they expected me to eat the cost. So I end up ordering some more vinyl, which ended up costing me more than the roll would have cost me.