AFIS is a good system I think, but pilots have absolute responsibility for their actions and what they do. In the air a FISO can give a FIS and ask for a pilot's intentions. They can't ask you to join in a particular way, they can't give you any clearance at all. It's all down to the PIC.
Without wanting to get into an arguement about CAP413 et al. (I know what it says), I see nothing wrong with a FISO
asking me to join in a particular way (or asking anything else) - I can always say no, after all. They've presumably got a reason to ask (maybe they have a student on first solo and my 10-mile airliner straight-in would make him have to orbit on base?), and if it doesn't cause problems for me then why not?
Telling me is, of course, a different matter (and would tempt me to reply "negative, Flight Information Service only"
).