Not getting passport stamps can become problematic. If something happens somewhere, like an issue with getting a departure marked off in an electronic system, you're going to need a stamp proving you entered another nation to show that you didn't overstay. eg, CBP doesn't get the I-94 departure sign off from the airline and thus doesn't have you recorded as having left. The next time you enter the US, they are going to think you overstayed unless you can show them a stamp that you entered another country within the time period your prior US entry allowed.
In 2011, I was at ICN checking in for a flight via NRT to DFW. The JL staff were refusing to check me in, claiming I needed a US Visa. I didn't. I was travelling under the VWP. The JL staff claimed I didn't meet the requirements for the VWP because I didn't have an e-passport. I didn't require an e-passport as my passport had been issued before the e-passport requirement. (Between 26 Oct 2006 and 25 Oct 2016, the rules of the US VWP allowed passports issued prior to 25 Oct 2006 to only need the machine readable zone)
If it wasn't for the US passport stamp from May 2011 showing that I had entered a few months before under the VWP, they would have denyed boarding.
If for some reason, a departure from FRA isn't recorded in the central EES database, there are going to be issues when you try to enter the EU again somewhere else. Immigration at CDG, HEL, MAD or AMS isn't going to know that you departed the EU within your allowed period if the departure at FRA 6 months prior wasn't recorded correctly, unless you have a stamp in your passport that you can point to.
www.independent.co.uk
