Evolution and Degradation of Language

So, at a wonderful BBQ at YKW's place, the conversation turned to language. I mentioned that the short people had discovered MadLibs at the same time I was grading student papers, and this led me to think about parts of speech. I remarked how the adverbial form of words is disappearing from colloquial (and formal student written) English being replaced with the adjectival form -- "He writes bad" or "She runs slow" -- and how even people whom one would describe with the word "learned" (when used as an adjective with two syllables) now almost universally employ the word "quote" (a conjugated form of the verb) in place of the noun "quotation," as in "I found a great quote to support that point."

Gwydion, whose livelihood is language, remarked that it should be seen not as a degradation of language, but as a natural part of the evolution. Language is a living thing, he argued, alive and changing in the usage of the linguistic community and that there is a natural and proper pull between linguistic conservatives trying to maintain some sense of purity and the forces of linguistic selection introducing alterations into the way we speak.

This is, no doubt, true. But to play on the evolutionary metaphor, surely some mutations are advantageous and others not. Biologically, we spot successful adaptations through successful propagation, that is, if natural or sexual selection are aided by them. Should we use the same standard for linguistic changes? Is the suffix "ly" like the appendix, simply a useless relic of a former time? Is "quotation" a dinosaur that is rightly going extinct? Or is there reason to be protective of parts of speech for sake of clarity, eloquence, or some other linguistic virtue that may, always or in some cases only, stand above utility, simplicity, or whatever else it is that is spurring these changes?

Are all changes in language evolutionary or do some leave us with an inferior language?