You care anything about weight? and especially weight at the tail? A straight taper which is very easy to do reduces the weight of the elevator by close to half. But that's not all.

Tapered aerodynamic things have less drag. Less drag is good, right? And they have measurably less drag. Good thing. But that's not all.

Squared versus sharp trailing edges create larger wakes. Larger wakes create lots more drag. Drag is not worth having if you don't have to have it. And you don't. But that's not all.

With some wing/elevator/rudder profiles, a tapered surface gives the modeler a "built in" exponential. The surface is less sensitive around neutral. Control line precision aerobatics found out long ago that untapered, square TE flaps often created a model that hunted in level flight and was a witch to fly smoothly in the level laps. But it's actually not really less sensitive, it actually is more accurate in exactly the same way exponential is more accurate. But that's not all.

A squared TE is easier to do accurately than a rounded one. It's also easier to do than a sharp one, but not by much. It's also easier for modelers to maintain. But the thickness of the block has nothing to contribute with it's thickness, so the best TE for most modelers is a obviously "thin", squared TE. And since it's dead simple and fairly easy to create a taper to a small squared TE, it's a no-brainer to have the benefits of both worlds.

But that's not all......

When you show up at the flying field with your tapered surfaces, the guys who appreciate good workmanship and sensible design will consider you to be a modeling god and heap praise and adulation upon you...... well, sometimes......