Generally not, but it depends on what you are doing with it. A good mocap system can easily set you back hundreds of thousands, and will take some time to get up and going. Finding experienced staff is difficult too.

The reason you would choose mocap should be an aesthetic one; you want to capture the performance of a real person and translate that on to your computer model.

It may be cheaper if you have a fairly high volume of realistic human motion needing done.

There are some cases where keyframe animation cannot provide the result, for instance for long takes of complex motion (e.g. martial arts). There are some cases where motion capture cannot provide a result, perhaps for motions that defy the laws of physics.

Motion capture is good for capturing the subtle details that can be difficult to animate, such as secondary animation, but is limited to that which is physically possible within the capture volume.