IPS will pick out things that AV wont flag like SQL attacks on a web server or scanning behavior. There may be a small amount of overlap in that IPS could find a signature for a well known virus and knock it out in transit but for the most part they go after different things.
Now is it recommend? Well its certainly not a bad thing to have but its really something you have to decide for yourself. Is what you are trying to protect important enough to warrant another layer of defense? Is it required for any compliance reasons? Is the risk of not having it acceptable?
The Cubicle Wizard