Task completion is not enough. How was the task completed? Was it accomplished in a quality manner? I think attitude will impact task completion. It will be extremely difficult for tasks to be completed in a quality manner effectively over a long period of time if an attitude is negative. There are also various levels of poor attitude from mildly upset to a willingness to openly damage the company in some manner.
Once I had an extremely talented young man working for me. However, he felt he should be in management and making the call on how everyone's work should be done. I had to talk to him about his attitude. He felt he was more talented than others and his attitude should not matter. He was very talented. However, as I had warned him, he was let go because of his venomous and contagious attitude.
If the task is to place a nut on a screw for eight hours a day, then maybe attitude is not as relevant as task completion. Beyond that though, attitude can impact not only the quality of work, but ultimately can be felt by the customer or client. This latter issue can hurt company by lack of customer or client loyalty, repeat business, and/or referrals.