In GA terms, the usefulness of a Long TAF is that one can plan the night before whether a flight the following day is likely or not.
The ability to look a day or three ahead is a much sought after ability, since e.g. many private pilots fly only at weekends and they need to "book" the weekend in good time, both with their other half etc and with the club or hangar owner.
For example I have to give a day's notice to get mine out of the hangar, and I have no weekend hangar access.
The only officially sanctioned longer range data is the long TAFs (which are available for major airports around Europe) and the MSLP charts.
TAFs are produced hopefully by full time specialists and should be the most accurate data you are going to get. The MSLP charts run 5 days ahead but are good for showing up major features such as fronts (OK for a go / no-go decision in most cases especially for VFR) but require expert interpretation to get anything else out of them with any reliability.
Many pilots use other data for longer range forecasts, e.g.
GFS.
Obviously if you are unemployed/retired, based on your own farm strip, and own your plane, and do only one-day trips, then you should never need a long TAF
