I'm a strong believer the anything one can do to minimize camera shake will help sharpen the image. I use monopods when feasible. They work great at figure skating events where your vantage point is often limiting and you can't readily move around to get a better angle. But, all to often, I find monopods more restrictive when trying to get the shot. For example, at grade school basketball games, I'm able to shoot courtside, but I need to be able to move quickly to stay out of the way or to dodge a player who's blocking my shoot.
In both of these examples, I'm typically shooting faster than 1/400 with a 70-200 f2.8L II on a 7D. Conventional wisdom suggests that higher shutter speeds eliminate the need for added camera shake prevention -- such as IS or a monopod. But, a 200mm lens on a crop body is like a 320 on full frame. The reciprocal rule suggests the 1/320 is the minimum safe hand held speed for this lens. 1/400-1/500 isn't that much faster. IS and/or a monopod will definitely help.
When it comes to candids, such as wedding receptions or event photography, you have to be quick to get the shot. Physical supports like a monopod will likely get in the way.
On paper, it may make sense to save money and use monopods instead of spending extra for IS. In practice, I think you'll lose more shots quickly tire of carrying the monopod everywhere. You may find some situations where a monopod is an acceptable alternative to IS for a given some lens. But, if IS is available for the given focal length and aperture, it will give you greater flexibility than the monopod.