I've been spending three to five months on the island for the last 20 years and never purchased an IDL. I have been stopped twice in roadblocks on the French side and showed a US based license (probably NY, earlier years NV). No problem.

I was stopped in vehicle controls on the Dutch side because my rental car operator was a bit behind in getting new plates. They let me go and I don't even remember if they wanted to see the license.

Having said all that here is a page from the French Embassy that basically says you need a notarized translation of your license and strongly recommends an IDL, which is something like a translation notarized by AAA.