If you have a high frame rate to start with it’s pretty nice and feels smoother. But a low frame rate turned into a high one looks good but feels laggy.
So arguably you never need frame gen for a game, since it only really works when it’s already pretty nice.
You will never ever get decent 1% lows in most titles, the software stack is architecturally fucked in the popular engines and can’t do it. You would need a CPU that’s literally 100x faster than today’s top models for it to be able to compile shaders on-demand within a single frame without hitching. (Or maybe it’s more accurate to say that there’s a massive gulf between what the hardware/drivers need - compiled pipeline objects built/known ahead of time - versus what game engines are doing, building pipelines on the fly on demand, surfacing new permutations frame-by-frame)
This requires knowing what to compile, which these engines don't really do, because the necessary data is pooped out by arbitrary game logic / scripts. That's why precompiling shaders in e.g. UE5 basically relies on running the engine through a pre-recorded gameplay loop and then making a list of the shaders/PSOs used; those are then pre-compiled. Any shader not used in that loop will cause stutter. A newer UE5 technique is to have heuristics which try to guess which PSOs might be needed ahead of time.
If you read their proposed solutions, it's quite clear they only have patchy workarounds, and the inability to actually pre-compile the needed PSOs and avoid shader and traversal stutter is architectural. It should be noted that these engines are also stuttering on console, but it's not as noticeable since performance is generally much lower anyway.
So arguably you never need frame gen for a game, since it only really works when it’s already pretty nice.