the FP unit often times cannot accurately record the difference. If your framerate is especially high, the FP unit may record the time difference as 0 and everything in your world will stop.
Not true, the clocks inside a x86 PC are accurate enough to get times in the sub millisecond range, otherwise a today OS couldn't work very well. Its also possible to grab the time as double precision floating point from the OS, being valid for about 10k years in the sub millisecond range, subtracting the old time from the current time and converting it to single precision floating point afterwards doesn't really reduce the accuracy by much.
