It's of limited use. The Gloranthan Visions fiction is still as
valid as it ever was. Not all of the material in the other books has
reappeared in other publications, although it may be on the web (e.g.
the description of Sir Ethelred's Black Horse Troop.)
On the other hand in most respects Hero Wars is an earlier version of
Hero Quest with some unfortunate problems. And the material that has
yet to be reprinted needs some adjustments to adapt to the new rules.
I'd say that the price is low enough to be worth considering but not
so low that I'd recommend buying it automatically. It really comes
down to what it is worth to you - and there are probably more
important books to get.