Why did he throw that?

[Gorwingel]

In addition for convienience such a heavy and obviously valuable item wouldnt seem my ideal choice as a projectile weapon.

I think it would make a fine projectile weapon. I have always imagined it as something looking like a bowling ball, and bowling balls can do great damage. And even though it was valuable, wasn't it also greatly indistructible, kind of like a diamond. Grima would have had to do much more to inflict any kind of damage to it.

Additionally I also think that Grima would have known something about it. If anything I think that he would have known at least that Saurman was getting information from it, and that it was also extremely important to him.