When we select a stimulus and we focus on that stimulus, we are paying attention to the stimulus. This question, then, is a question about whether it is beneficial or detrimental to pay attention. Clearly, there are benefits to paying attention, but we must not pay attention too closely to a specific stimulus as it can blind us to the importance of other stimuli in a given situation.
One situation in which paying attention can be detrimental when done to excess is driving. Let us say that I need to change lanes. It is very important that I pay attention to what is in the lane that I want to move into. However, if I fail to pay attention to what is in front of me, I might crash into a car that stops or slows down while I am trying to ascertain what is in the lane next to me. In other words, I have focused excessively on one stimulus (the activity in the next lane) and that has caused me to fail to focus sufficiently on another stimulus (the car in front of me).
As another example, let’s say I am doing an academic research project. I decide that Factor X is the most important thing for me to focus on. But this can be bad because it will lead me to ignore the impact that Factor Y has on the thing I am researching.
In short, focusing on a selected stimulus (paying attention) is very important. However, it is also very important not to focus to an extent that is inappropriate to a given situation.