As for serious routes,
I would say that John's wrong to say that serious routes don't get star recognition in the book. Free Finale, Seventh Sojourn, Highway 61, The Arete, Stage Fright, Clean Sweep, Toothless Grin, South Buttress Direct, London Calling... to name a few serious routes that get 3 stars...there are lots more...if anything I probably tend to be a little biased towards those routes because they left such a big impression.
As for easier routes,
its probably true to say that the real strength of North Conway climbing lies in the upper grades, but its useful to users of the book to differentiate between routes at a given grade to a certain extent. So, there are probably a few ok-but-not-great easier routes which get a star or two that aren't really comparable quality-wise to harder routes with similar star ratings.
As for dirty routes,
in an area like North Conway with a lot of small obscure cliffs spread out over the region, I think its worthwhile to give great routes a lot of stars even if they're grubby because of few ascents. At least that lets people know that its worth the effort to walk out there, even if they have to clean something up. I can think of loads of examples of this, including quite a few of the routes at Owls which were starting to get a little grubby when I was last there. Its just a fact of life of climbing up here for those interested in repeating 5.11 and up routes.
Obviously, its not an exact science, and for every route you try and weigh a bunch of different factors...with the most important being JH on the FA line...thats always worth at least a few extra stars!