TPA Controller failed to provide Class B exit information to an Air Carrier; noting the exit requirement is unnecessary and is a safety factor.

Narrative

A B757 was on a left downwind for Runway 1L at TPA descending to 3;000 when I took the position. I initially turned the B757 to a 100 heading for a base leg and descended to 2;600 (this is the altitude at which MCF must be crossed before descending for the runway). After doing so; the relieved controller pointed out to me that the B757 had descended below the Class B airspace without being advised. Due to the seemingly pointless construction of the Class B airspace in this particular area; as well as the seemingly pointless requirement to advise aircraft when they are leaving Class B airspace; I forgot to advise the B757 (in addition to almost every other left downwind traffic landing TPA that I have worked). Honestly; I think the requirement to advise certain aircraft they are leaving Class B airspace is preposterous. It not only inhibits the Controller (as they attempt to keep aircraft in the Class B to avoid slowing); it also adds undue verbiage and workload. There are several locations within the Tampa TRACON airspace where this is a factor; but this particular location on the left downwind for a north operation needs to change. Recommendation; change the requirement to advise aircraft leaving Class B airspace. At least in the vicinity of TPA; this requirement is less safe. Either that or change the Class B airspace.