And.. it is a weakness.. a big weakness. it is no bullshit.
1. It's a common stock answer, so giving it signifies a lack of imagination.
2. Taken at face value, it's a real weakness (good; honest), but it's one that can cause problems for the business (bad). It's a crippling trait which might prevent a person from meeting deadlines, and that's not something you want to indicate during an interview.
You want to answer the "biggest weakness" question with a genuine weakness (not "I work too hard") but one that can't, in any way, result in missed deadlines or upset clients/co-workers. This means that most people need to "lie" to the extent of selecting their #3 or #4 biggest weakness. The actual biggest weakness, for most of us, won't get the job.