Very, very, very important.
My only bread-and-butter orchestra is EWQLSO, the gold version, and every time I use it I feel like an alchemist, mixing reverbs, comps, etc. like crazy, so nowadays I only use the solo patches most times.
That limitation taught me a lot back in the day, but it also made spend twice the time doing that kind of wizardry than improving the composition or the performance.
It may also be one of the main reasons I began exploring other musical realms (still orchestral, sort of, but not tied to the idea of what a real orchestra would do) so, in some ways, it opened my eyes to awesome posibilities. But really, it was painful to get the result I was looking for.
On the other hand, every time I open a Tundra patch (I know this is a peculiar lib, but still) I play with the mics, and maybe the panning, sometimes too much, and then I just focus in the composition and the feelings. I can't picture myself doing the same things with only one mic and not destroying the sound quality.
I'm sure the same could be said for most libs from most developers, EW included, but I don't understand the EWQLSO upgrade price...