GMs really think like that -- and have thought like that for a long time. This isn't all that radical of a thought process. Look at it like this: There is an aggregate amount of offense that a team needs to get to have a run differential to contend, right? That offense has to come from eight positions on the field, however, not all positions are created equal when it comes run production vs. run prevention. For the most part, production comes from the corner positions, right? Look at what good teams get from first base. That's where giants tread, and that's one of the big deficits the Cardinals have had since 2011. Third base is often an offensive position, as are the corner outfield spots, especially LF. However, there is a sliding scale and we see that a lot of teams. If they have a standout second baseman when it comes to two-way play or offense, then they can give a little elsewhere. If there is a two-way shortstop then a team can maybe go with a good-glove second baseman that doesn't have to do much offensively to stay on the field. All teams take into account this sliding scale of production. It's why glove-only shortstops can have long careers with teams that have MVP monsters at the corner. Omar Vizquel was such a gifted defensive player at shortstop -- but he wasn't alone in that gift. He had the benefit of being on a Cleveland team with mashers galore that allowed for Vizquel to play shortstop, improve as a hitter, and stay on the field. Same could be said for a young Yadier Molina -- who didn't need to do much offensively for a team that had Rolen, Edmonds, Walker, Pujols, and so on all around the diamond.
Think of it this way: There are two lines like this ...
Run Prevention --- minus-3/minus-2/minus-1/ZERO/plus-1/plus-2/plus-3
Run Creation -- minus-3/minus-2/minus-1/ZERO/plus-1/plus-2/plus-3
As a team you want both of these to be as much plus as possible, and if you have a shortstop that is a plus-3 RP and a minus-3 RC, then you can get into the overall plus with a third baseman that is a plus-2 RC and a plus-2 RP. Makes sense? That's what I'm talking about. And, yes, teams do think in these terms because they need the roster, as a whole, to add up to a total amount of production that would be a contender.