A few days ago an article I was subediting had preventive and preventative in quick succession. Dictionary.com’s main entry is preventive, but it adds “also preventative”. Its entry for preventative redirects to preventive. Google Ngrams shows that preventive is more common, but preventative is certainly an established alternative. So preventive it became.
I have been browsing through Jan Freeman’s discussion of Ambrose Bierce’s Write it right. He states: “No such word as preventative”. She adds: “There was and is such a word as preventative, of course; it arrived in English only a few decades after preventive, in the mid-17th century. Two hundred years later, Hurd 1847 [Seth Hurd, A Grammatical Corrector] decided that the longer spelling was “a common error”; it took another couple of decades for usage critics to declare it nonexistent. Preventative is longer and less common than preventive, and no doubt is a “needless variant,” but its enemies have not yet managed to drive it out of use, or out of our dictionaries.”
If you want to choose, or have to choose, use preventive: it’s (two letters) shorter and more common. Otherwise, be consistent.
So what is it about preventative which had usage commenters of earlier times foaming at the mouth? There is a long list of adjectives ending with –ive, –ative and –itive. Almost all words ending with –ative have corresponding noun ending with –ation. There is “no such word” as *preventation; it’s prevention. The most analogous word is invent > invention / *inventation > inventive / *inventative.
So why did people start and continue to use preventative? I don’t know. But “no such word” or “not a word” is not an argument, certainly not by itself. If some people use a word, and everyone knows what it means (even as they are foaming at the mouth), then it’s a word.