B is the key here.
The problem is that number of hours is too often thrown around as a comparison between two different people.
I'd argue that arises from the more frequently asked question of "Is employee A more productive than employee B?" rather than "was employee A more productive on Tuesday or Thursday?"
Employers certainly ask the first question when they're looking for salaries to dump.
How much time you spend is correlative with productivity. Most of my really productive weeks are where I log an unusual number of hours in my dev environment. I think the relationship between # of hours and productivity is especially strong when:
A) Looking at a longer period of time (certainly there are raredays where you're in TextMate all day but don't get anything meaningful done, just as there are days where you destroy a critical problem in 30 minutes)
B) You're comparing to yourself. Comparing your work-month to mine isn't very meaningful. Comparing a month of my data the middle of our YC experience versus a month of data in the middle of our fundraising efforts vs a recent month of data is interesting.
But I also think that the sheer # of hours is a really interesting measure of engagement, motivation, and workload. If you're only really working for 2 hours a day, chances are the issue is with that rather than with your ability to be productive.