It does not render the unused pitcher useless by just warming up once,
but it does plays a role in his ability to pitch effectively. Not only physically, but mentally. (There is also a difference in "warning up" and "getting loose".) If he is the guy Not used, do you commit to him the second time? What if the guy that does come in lasts 2 or 3 innings? Do you keep the second guy warming the whole time? (Some relievers have done well the inning they come in, only to blow a tire the next inning.) If not, he will possibly have to warm a third time. Does this affect him mentally? If so, how much? Is he a freshman or a senior? That could play into how tough he is mentally. (And also explain why upperclassmen seem to handle this better later in their careers. Remember, all of these guys were the studs coming in. Now they may be 4th or 5th choice. Think only having a chance to pitch once a weekend puts more pressure on the kid to succeed? Fail and you may not travel or be on the active roster next week. Fail a couple of times and you may not be on the roster at all next year.
And by the way, different coaches have different ideas on all of this.
One other note. There is no way this year to get innings in games. (Remember the "mid-week guys"?) The lack of mid week games and 4 game conference weekends changes the way pitchers were brought along. Few chances to help a guy get his feet on the ground.
[I always wanted to have a job where my ability to keep my job and provide for my family depended on 18 to 20 year old kids.]