What is the 'end plate' effect?
An end plate on a lifting surface (which the vertical tail is, during a yaw maneuver) reduces tip vortex formation, which means less lifting area is needed to produce the same lift force.  It also significantly reduces induced drag, though the parasitic drag of the plate is usually a net loss unless, as in the case of a horizontal tail, the "plate" serves some other function that would be required anyway.

Whitcomb and newer design winglets use careful design to give similar effects with less, or even negative drag -- but you'd need a horizontal tail anyway, so the reasons not to have a T configuration boil down to structural weight, elevator blanketing in a stall, and control routing issues.

For an example of an aircraft that makes good use of tip plates to allow a shorter wingspan, look up the Bumblebee -- at least at one time, it was the dimensionally smallest piloted aircraft ever flown, and it critically depended on tip plates on both upper and lower wings to produce enough lift to fly.