ive had this discussion quite a few times with my buddies, i think the answer is what most the replies have been thus far: sometimes you should, sometimes you shouldn't. i used to push until i was barely concious of my surroundings on every work out, then i decided to mix it up and try to improve time and scores, got some good results. however i noticed when i did put in maximal effort i would get naseau and dizziness much sooner.
the maximal efforts seemed to have the effect of increasing my work capacity for a 3-5 minute period, which in terms of real life application are extremely important. Think of a situation in which a 1-5 minutes sequence of intense varied activity might keep you alive to see the 20 minutes you will need to recover.
i say use crossfit mentality and mix it up.
sorry about the grammar and confusing wording, i had to rush through this one.