FX loops: When does an amp need one??

[makinrose]

Over the years I've installed a number of mixing FX loops based on the TUT Best All Tube FX Loops with a lot of success and find that design to be transparent and more useful than the serial loops in many amps.  I like the idea that the dry path isn't going through all the buffers, mixing circuits, etc. in the pedals as well. 

In general, I've only thought an FX loop is needed in higher gain amps but more and more I'm seeing them lower gain amps (Bassmans, Plexis, etc) and get requests to install them in.  At what level of gain is there enough benefit to warrant the effort?  I was hoping I could get some guidance  and opinions on the matter.  Thanks everyone for the help!