I wouldn’t say it is useless, it all depends on where the main source of interference is.
I got a good improvement using the original filter over the ProStick plus on its own. This was because it removed very strong broadcast FM and DAB that despite the filter in the ProStick plus was still causing a problem. Here are the graphs from when I added the original filter in May last year.
I further improved things later on by removing the original filter and replacing it with a cavity filter that also removed the local mobile phone signals, but the original filter was a really big improvement over the ProStick plus on its own.
Currently I have 2 stations under test, one a ProStick plus, the other an airspy mini. Both are fed from the same antenna system with an RTL-Sdr LNA. The LNA is powered by the airspy mini and the ProStick plus is fed by a 20dB directional coupler to account for the gain difference. The airspy performs a bit better than the ProStick. The airspy however is running on an odroid xu4 the ProStick on a raspberry pi.