Although I now use pencil jets, I started off with fan jets as supplied by Peter, worked great, no problems at all, well designed brush contained the spray.
I now use pencil jets, but my brush is also smaller than the original brushes I used from Peter and I prefer pencil jets in them as I can operate with a much lower flow rate.
To get a good 'fan' you need good pressure.
The rinse on rinse off debate doesn't bother me, I use the bentley superlite brush and rinse on and get absolutely perfect results.
The Bentley is a small brush, it needs two pencil jets to operate effectively, but two fan jets would be overkill because of the pressure/flowrate needed for the fans to operate at their best. and I don't think a single fan jet set in the middle of the brush would be effective enough.
On a larger brush you would need at least 3 pencil jets, but you only need 2 fan jets. Horses for courses methinks.
Peter makes good fan jets, perhaps I'll get a couple off him and stick them in a Bentley brush and compare like for like.
I know you can of course turn down the flowrate and use a fan jet by the way, but I think they operate better with a high flowrate.
Ian