Let me add my interpretation to what is quite a contentious topic......
BMA123 Descend WHEN READY FL260 level by margo
In this above example I think it is perfectly clear that the instruction states that the pilot can start his descent when he is ready as long as he makes the restriction by margo.
BMA123 Descend FL260 level by margo
Now in this example the pilot should be expected to vacate his level and start descending at or greater to the minimum requirement ( 500ft per min ) to be at FL260 at or before Margo
To me this is pretty much self explanitory. If you don't say " WHEN READY " or words to that effect you should vacate the level immediately.
This topic is exactly the reason why more pilots should be visiting ATC to see exactly what it is like on the other side and why us ATCers should be getting fam flights
