Cusco,
I don't know if there is any valid data on the number of microlight pilots that are non-radio.
It is true that many weightshift microlight radio installations are of a permanently powered hand-held (ie Icom) type, and that the hassle in getting a radio installation licence means that many may well be flying with un-registered radios.
So they will show up as non-radio when they are not.
(if you rent off a flying club, you've probably never seen the paperwork you have to fill in as an owner!)
It is also true that you do not need to hold a FRTOL to get a NPPL (Microlights) - and possibly not even for an NPPL (SSEA).
So - unlike the US where anyone can use a radio without passing a test - some microlighters may be banned from transmitting on their radios, even if they own them just for listening out.
Use the Freedom of Info act to ask the CAA the number of new microlight licences issued to non FRTOL holders.
And then there are some people - microlights and light aicraft - who have a radio problem but still want to go flying on a CAVOK day.
And there are some commercial pilots - I know of one Islander operator - who likes a bit of piece and quiet and choosing his own routes without ATC when at all possible.
So, what percentage of pilots - microlighters and others - are non-radio?
Very best,
XA