It really all depends on how much patience you have. Using Motion as a stand alone editor is possible, but it is an incredible hassle. For example, cutting film into different pieces puts those different pieces into different layers. Sometimes it detaches the audio, which then means you have to do audio syncing. Plus, if the project is a long one, your machine might start to choke on it. I wouldn't recommend Motion as a stand alone editor.
But, that being said, I'd still recommend Motion for what you're talking about. I think it is stand alone enough to do what you want, all you really need to do is get used to round tripping between iMovie and Motion. To do that:
1) Cut your movie in iMovie
2) Export it and import it into Motion
3) Find the part that you wanted to put titling on and use "split" to separate it from the rest of your movie.
4) Delete the part you don't need.
5) After you done using Motion, export the movie and import it back into iMovie.
6) Just put it into the place it had been before...
I really like Motion myself (use it all the time), and it certainly can add that extra zing to your movies.
One more piece of advice, go find the Apple Training for Motion 4 by Mark Spencer. Great book, easy to use and gets you up to speed really fast. Yeah, I know, it's for the old version, but Motion 5 is literally the same thing as Motion 4 with the addition of rigging. And if you're not using FCPX you don't need rigging. So, go get the book :-).