Yeah I'm well aware of the problems when having unstable framerates, but I'm still not sure what the ideal solution might be - just making everything framerate dependant (and account for delta times) might not actually be the best solution in any case, even if it makes increasing (or lowering) the framerate rather simple. For one, there are issues with floating point precision, especially if you go really high with the framerate (from my memory Quake3 had that issue, there were "magic" frame / refresh rates that gave players an unfair advantage, so Carmack decided to cap logic updates at 60 fps for Doom3 Multiplayer), and when the framerate goes really low you risk collision bugs, wrong approximations and other nasty things.
For multiplayer, I guess it would be ideal to assume a (more or less) constant update of the network data (say 30 times / sec), and it should be inter- / extrapolated for each drawn frame, with special care taken for local prediction.
So those two examples suggest using a constant game logic refresh rate, but that leaves the problem of handling inter- / extrapolation (assuming that rendering is running asynchronously with varying framerate), which means more controller input lag (ideally you would want to poll the controllers the very last moment before you render view dependent stuff).
And then there is a third possibility, that nowadays probably is viewed as outdated or obsolete: you lock your logic to the framerate without any adjustments to delta time (so you can have perfect integer / fixed point arithmetic that works ideal / without any numerical artefacts) and do your damn best to keep the framerate above your chosen number (obviously also capping it so it runs "locked"). Yes, this means that if for whatever reason the game fails to update in 1/60s, things will slow down. But on the other hand, you have that pixel perfect, snappy feel where pixels move at a constant rate (ideally also constant speed if framerate is 100% stable), making single frame & pixel precise jumps / moves etc. possible that nowadays are simply not possible anymore due to triple buffering, extrapolating, floating math, upscaling and whatnot..
But I fully understand that it's nearly impossible to recreate that feeling on today's hardware with all its abstraction layers and multitasking, and I also understand that most people don't have the patience anymore to really nail moves with 100% accuracy - might get flamed for it but today's games are simply much much easier than they were in the past, and they have to be easier for 2 reasons (imo): the broadened audience isn't willing to work so hard to beat a game anymore, and today's hardware simply isn't capable anymore of truly precise timing / input / display.
/rant of a grumpy gamer & part time dev