Standard procedure in the U.S. Saves us time because we can clear you when we have R/T time and not have to wait/watch and clear you after a preceeding has actually cleared the runway. If spacing and relative speeds look good, just do it. On the other hand, if I think it's going to be tight, I'll always advise the pilot. From the U.S. ATC Handbook, 7110.65:
3-10-6. ANTICIPATING SEPARATION
Landing clearance to succeeding aircraft in a landing sequence need not be withheld if you observe the positions of the aircraft and determine that prescribed runway separation will exist when the aircraft cross the landing threshold. Issue traffic information to the succeeding aircraft if not previously reported and appropriate traffic holding in position or departing prior to their arrival.
EXAMPLE-
"American Two Forty-Five cleared to land, number two following United Boeing Seven-Thirty-Seven two mile final, traffic will depart prior to your arrival."
"American Two Forty-Five cleared to land, number two following United Boeing Seven-Thirty-Seven two mile final, traffic will be an MD 88 holding in position."
Besides, if the aircraft involved are light piston singles or twins, it's quite legal over here to have two or more arrivals on the same runway at the same time...
I should have also added, that in low vis ops when I can't see the aircraft exiting the runway, I will, of course always ASK the preceeding aircraft to report clear. You should be able to hear the report as well as I. ASDE radar negates the need for the report natch....