From AnArchitecture, quoting an article from Inside PR.

“I actually hate billable hours. Don’t get me wrong – I like working – I really like bringing in the money.. but I just don’t like tracking every minute of my time – especially when it comes to multi-tasking and working on more than one thing, or often a lot of things at once. I also think billable hours aren’t a good measure of the value people bring based on experience but may just take an hour to create. I know the reason why there are billable hours and why we need to track our time and profitability but I wish there was a better model.” source