I value a fuel shutoff valve more for what it does and how easy and safe it makes common task and I don't worry a bit about it leaking.
If it leaks fix it, same as any other leak.

I thought you were asking for a good reason(s) to have one... When you asked did you already know there wouldn't/couldn't be any reason good enough for you?

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-