From a business level google's job is to sell ads, it sell ads by providing valuable search results 'for free', and one part of the intrinsic quality of the search results is that they are 'good' in the sense that you describe.
I think this gets to the heart of the conflict of interest which Google experiences and those who depend on them are afraid of. On the one hand, they have a reputation for valuable and high-quality search results which are not vulnerable to gaming. On the other hand, they have an established monopoly on search, which will not disappear overnight, and a business selling ads which compete against their own search results for clicks. Their interests do not currently align with those who appear in their organic search results, except tangentially and in the long-term (as a way of keeping eyeballs on those results because everyone views them as trustworthy).
So there is a tension there as good search results mean less ad clicks short-term, and less ad clicks means less money for Google. In an ideal world for Google as a corporation, their first page would simply be search results, tailored to a user's request, with no organic search results whatsoever, and indeed, their space on the results page does have a growing advertising component (paid ads, G+, ads for google places or other services like shopping). Obviously at present Google has managed to walk that tightrope and provide great search results, downgrade spammy results and ignore bad links, while making money from advertising displayed with those results, but it's a delicate balance, and there are commercial pressures to improve advertising clicks, but no short-term commercial pressures to improve search result clicks.
If those results can be poisoned or gamed easily, it doesn't really affect Google in the short term, only in the long term, and only if users start to notice and desert their service (over a period of years probably) - that's not necessarily the case if users don't have many comparison points because of Google's scale and dominance. It might even be good for Google in the short term, as businesses start to buy more adwords visits when their organic results decline. I'm not trying to suggest there is some conspiracy at Google, just pointing out the intrinsic tension at the heart of their business between good results and good ad revenue.
I think this gets to the heart of the conflict of interest which Google experiences and those who depend on them are afraid of. On the one hand, they have a reputation for valuable and high-quality search results which are not vulnerable to gaming. On the other hand, they have an established monopoly on search, which will not disappear overnight, and a business selling ads which compete against their own search results for clicks. Their interests do not currently align with those who appear in their organic search results, except tangentially and in the long-term (as a way of keeping eyeballs on those results because everyone views them as trustworthy).
So there is a tension there as good search results mean less ad clicks short-term, and less ad clicks means less money for Google. In an ideal world for Google as a corporation, their first page would simply be search results, tailored to a user's request, with no organic search results whatsoever, and indeed, their space on the results page does have a growing advertising component (paid ads, G+, ads for google places or other services like shopping). Obviously at present Google has managed to walk that tightrope and provide great search results, downgrade spammy results and ignore bad links, while making money from advertising displayed with those results, but it's a delicate balance, and there are commercial pressures to improve advertising clicks, but no short-term commercial pressures to improve search result clicks.
If those results can be poisoned or gamed easily, it doesn't really affect Google in the short term, only in the long term, and only if users start to notice and desert their service (over a period of years probably) - that's not necessarily the case if users don't have many comparison points because of Google's scale and dominance. It might even be good for Google in the short term, as businesses start to buy more adwords visits when their organic results decline. I'm not trying to suggest there is some conspiracy at Google, just pointing out the intrinsic tension at the heart of their business between good results and good ad revenue.