I'd have to say both.
I think chat clients are indispensable as they've been a good tool to have. Frequently I'd need to chat with one or more people in real time to confer on projects, exchange notes, etc, and being able to exchange links, data, and files quickly and efficiently helps with this. Email does not offer this.
On the flipside, leaving chat programs running all day often can mean frequent interruptions, distractions, and pestering that makes it hard to get anything done if they aren't managed well.
So it comes down to if the user is able (or had the forethought) to keep his "business" clients separate from his social clients, then they can be more of a help than a problem. Personally I know some people that couldn't get much done without them since they often do half of their work not actually from their office.