Why not?
Simple, mis-identification and the amount of RTF needed as both pilots must comply. There are a few nationalities I wouldn't even consider asking due to misunderstandings too.
The mis-idents can come from something as simple as BMI aircraft flying using Lufthansa callsigns - as ATC we do not know if the aircraft we are speaking to is actually owned or even painted up in the callsign colours. I know we don't have to pass operator just aircraft type, but if you heard me call a Lufthansa requesting for you to visually climb through and then you noticed it was a BMI craft, you would feel uneasy and possibly question whether you are climbing through the correct aircraft's level. Too dangerous and negates us actually having radar and in the UK as many sectors have lots of climbing/descending aircraft in close proximity it just wouldn't work.