Significance of 3PP in general, in the minds of people in general is the answer to that.
Ah, that changes things. Since 99.9% of people in general have no idea what a 3PP is, I'd have to go with 'underrated' then.
I think the whole 'Open Gaming Content' idea is fairly amazing, and wouldn't mind seeing the model expand to various other media. Creative Commons takes a similar view, as does open source software, but the idea has yet to really take off and most copyright holders don't really have a set policy in place regarding 'allowed use' of intellectual property (both 'for sale' licensing and more freestyle 'third-party' works like fanart or fanfic or
'gaming days').
Can you imagine someone like J.K. Rowlings (who is fairly unsympathetic towards people using her characters) adopting a more 'open content' stance and setting guidelines for fair use of her intellectual properties?
Other creative types, such as Joss Whedon, already have a laissez-faire attitude towards fanfic or even published works involving his creations, but without a clear 'Open Licence' concept, he's obviously taking his chances on some twit attempting a copyright challenge.
I mistook the original question to be about gaming and specific 3rd party publishers (such as our hosts here at ENWorld), not about *people in general* and the whole 'third-party publishing' concept.
In retrospect, that was a silly assumption on my part. It seems unlikely that someone would post to ENWorld talking down the significance of their various contributions to D&D in general. (And talking down the significance of 3rd party publishers like Monte Cook, or authors who have worked for 3rd party publishers like Skip Williams (Green Ronin Advanced Players Guide), Ed Greenwood and Gary Gygax (various modules and books for diverse 3PPs)).
My apologies for the confusion.