It's pretty obvious why they would want to destroy the Endor base.

With the shield down the destruction of the Death Star is almost a certainty, therefore using it to destroy the moon is a spoiling move, destroying the Rebels on the moon, and also any ships in the area (almost every ship the Alliance has).

Sure the Imperials would take further losses but they have a lot more ships than those assembled to hem in the Rebels.

Han speculated that it would take a thousand ships to destroy a planet with more fire power than he had seen before.

Dodonna says the first Death Star has the same fire power as half of the Imperial Starfleet.

So at the time of ANH the Imperial Starfleet is roughly two thousand ships including many command ships like The Executor.

As well as The Executor there seems to be about two dozen Star Destoyers at Endor (give or take).

So the Rebels would stand to lose a lot more by such a move than the Imperials.