I'm trying to monitor an extremely volatile network link that requires 1 - 2 second polling to accurately capture peaks. On this link a scanning interval of even 5 seconds results in too much smoothing of the curve and does not give an accurate picture.
Beyond that, the pre-defined scanning intervals only go as low as 30 seconds. There are other applications where I might want 10 second or 15 second intervals.
The old version of PRTG supported scanning intervals of 1 second and complete freedom in choosing your own scanning interval.