Chilli,
When you say 'effectively putting themselves IFR' do you mean from a controller's point of view or the pilot's? I cannot speak for controller, but from a pilot I remain 100% VFR effectively and legally. For example, I am responsible for visual separation, and following visibility/cloud clearance regs. If a radar vector in Class B given by a controller in a windowless room in a different county takes me towards cloud, I would simply aviate navigate and communicate. No different from what I would do on a practice approach.
I think the 'maintain VFR' phrase is there to make sure there is no doubt that I am VFR, that should be a given, but no harm in reinforcing the fact.
The controller will always ask how the approach will terminate, then the pilot can state his intentions.