I found that keeping a log took away a lot of the enjoyment of fishing for me. It became work, and I found that I was going fishing to feed the log, not to have fun. And in 35 years of flyfishing I have never needed a thermometer, never felt that I was missing anything by not knowing a number. Am I less of a fisherman as a result? I certainly don't think so.

To me, a fisherman's best tool is a mind capable of paying attention to what goes on around them and adapting to the situation. If a log book and thermometer help with that, more power to you, but not everyone needs them.