Moonslayer,
Tower controllers use anticipated separation during high workload periods at busy airports.
This means that they may issue a landing clearance to an arriving aircraft when there is a departing a/c on the runway if they anticipate there will be the required separation by the time the arriving a/c crosses the threshold.
In the case of a departing and arriving jet transport this means the minimum required separation is the departing a/c must be 6000 feet down the runway AND airborne before the arriving a/c crosses the threshold.
The "rolling" call, while non-standard, is appreciated by the tower controller because it gives him/her the signal that the anticipated separation will work out.
At many airports it is very difficult to determine when a plane has commenced it's take-off roll due simply due to the distance of the threshold from the tower, or as stated previously during LVP's.
Without the "rolling" call the tower controller may issue a "Cancel Take-off Clearance" instruction to avoid having two a/c airborne in close proximity (one departing and one "going around") if it appears the departing a/c is not moving and the arrival is getting too close.
So IMHO, the "rolling" call, while non-standard, makes sense.