The 'Descend at xxx FPM or more' or 'be level in y minutes' are the bain of our lives at London because we get them all the time from certain watches/controllers at Maastricht. The flaw in them is that they are based upon an assessment of the traffic's performance at high level, and take no account of changing TAS or G/S in the descent. Often the traffic goes in high. The 'level 15NW of COA' or whatever we have as a restriction is to keep traffic out of S13's airspace and it can be embarassing if the assessment is wrong. Most of the pilots I have asked say they would prefer to be given a point by which to be level, and let them work it out.
I must confess that I used to be one of those who assumed a certain amount of common sense, especially amongst our home-based airlines who fly the routes regularly, and expected that they would comply with previous restrictions on the way to new ones. Since the exchange on here, however, I now restate the restriction if I think it is necessary. The wonders of PPruNe, eh?!
As an aside, I don't think I'm alone in the following:
Say I give you an intermediate descent: "Descend when ready FL310, expect FL250 by LOGAN". This will be against other traffic. If I then step you down in stages to FL250, I would expect you to make FL250 by LOGAN without me NEEDING to restate it every time. You were given the restriction in the first clearance. If of course the stepped descent has meant you CAN'T make it, then I will remove the restriction and place a new one (e.g. expect FL150 by SABER).
My point is that I don't want to take up R/T time by repeating restrictions in every transmission. You are professionals and I think it is fair to assume a certain amount of nouse!