I suppose membership of a "professional institution" other than IPEM doesn't automatically guarantee that individuals are eligible to be included on a register such as this.
The majority of individuals who are included in the VRCT will have filled in this form; irrespective of their "professional status" outside of IPEM.
The register is quite specific to a group of Technologists involved with Physics and Engineering in Healthcare and is maintained by IPEM. IPEM "makes the rules", so to speak.
It's been put to me that registering voluntarily might make the transition over to mandatory state registration for technologists (which is looking more likely) easier; particularly for individuals currently working in this field.
The costs involved with registering are fairly insignificant and the application is not a difficult form to fill in then I do not see what the problem is, personally speaking.
You did not indicate this - but if you're having trouble finding proposers and seconders then perhaps IPEM can give you advice, rather than IIE.
Of course, registration is voluntary, so there's no need to complain about it if you don't want to complete the application!