G.W.H.
I take your point exactly - the reason why I dont use conditional clearances when using stop-bars. At my airfield we only use stop-bars at night and in LVPs, the times when, IMHO, you need positive control i.e. I am clearing you to line up and I am switching off the stop-bar to confirm this. If I dont switch off the stop-bar I expect the pilot to query this and not move until I have done so. Using stop-bars and conditional clearances is in my opinion a contradiction.
M609
On our stop-bar system, you only get a time limitation when you have the LVP button selected. In these circumstances we require positive reports from pilots that they have reached a particular hold - we then de-select the stop-bar when giving line up clearance. This removes the problem of activating the button too soon.