I'm a trademark lawyer. There are two major risks in not consistently enforcing your trademark rights: (1) your trademark becomes the generic term to refer to the class of product or service and (2) when you do try to enforce them, the defendant points to all the other unauthorized uses you permit and argue that your rights are weaker as a result.
With regard to #1, it's pretty rare that a trademark becomes generic. Google isn't yet becoming genericized, because nobody goes to run a search on Bing or Twitter or Yelp and calls it "Googling" something. We use "Google" as a verb a lot, but generally people are talking about using Google's service when they do so. MLB's marks weren't at risk of becoming generic just because they were being used in the titles of podcasts about the teams.
#2 is somewhat legitimate, but it's hard to quantify how necessary it is to enforce against all unauthorized users. I work in-house for an organization whose marks are used without permission all the time and we don't have the resources to enforce our rights against all the random internet uses. But when a potential competitor uses our marks, or an unauthorized use that could damage our reputation comes to our attention, we enforce.
