It suffers from what a lot of metrics offer: it's easily measured, easily gamed, and not particularly correlated with output.

I'd say it's probably useful to measure, but also key to keep in mind when you're warming a seat just to move that number up, rather than actually getting things done.

Including other measures of work output would be a very good modification.

That's a really good question. I thought about this and for now its a fair metric. I want to reach 8 working hours a day. Once I hit that goal consistently, I'll have to look for a better metric.