I'm pretty against this. It seems a backward justification: if your intent is to prevent degrading the user experience, then implementing a framerate cap will degrade the user experience on high-end devices that are perfectly capable of 60 FPS. Give it a couple of years and then everyone's got more powerful devices which are capable of 60 FPS, and you're unnecessarily degrading the experience for everybody. So I don't really buy that this improves the user experience.
With delta-time variable framerates should still continue gameplay at a steady rate. Perhaps you could provide an in-game option to reduce the number of sprites/effects to a "lo-fi" version to help it run faster if the player doesn't like the variable framerate. Also I think most casual players are far less sensitive to framerate issues than we are - I remember a few years ago crappy software-rendered Flash games staggering along with a horrible framerate still going viral all over the web... and once I saw someone on a plane playing a Bejeweled clone at 4 FPS... completely rubbish, but they didn't seem to care!