As has been alluded to already, it's not just an ATC thing but also to provide additional safety to other aircraft that are fitted with ACAS.
Of course, this might be less of an issue if we established controlled airspace to protect commercial passenger carrying aircraft as many other countries do (instead of, sometimes, forcing them to fly through class F or G airspace). But then, whenever additional controlled airspace is proposed one of the main opponents is the GA and sport flying lobby. The other main opponent to more airspace is usually the mil - and if I recall correctly many BritMil aircraft have recently been equipped (as some expense to the taxpayer) with ACAS compatible equipment....so the mil are probably in favour of the idea of mandatory carriage of tx'ders to provide additrional protection to their fleet.
This is not to try and say who is right or wrong but simply to illustrate that it's a complex topic.
But resistance is probably futile because I think the proposal is intended to harmonise tx'der carriage and operation across the European region (I stand to be corrected by those who've read the proposals and their justification).
If you simply want my personal ATC view, for those controllers working outside controlled airspace it's a very good idea, and for those that have the luxury of only having controlled airspace to worry about it is a useful improvement to awareness of infringements etc.