My writers group got into filter words when one of our members started writing first person present tense. There was a lot of "I saw" and "I noticed," which does keep the reader at arm's length instead of riding right in the narrator's head, which is the whole point of FPPT. As for keeping filter words out of rough drafts--fine, if you're training yourself never to use them at all and don't mind writing slowly. And yeah, the editing will go quicker if you aren't weeding out all the saws and felts and rejiggering a bunch of sentences. Otherwise it's just going to gum up your creative process.