The thing is that in case you wnat him to play (for example) PG as starter, SG as backup and PG as reserve, there is a chance that he will not get (I believe) that 48 minutes, and I believe that he should have also on that case.
No, I would not necessarily want the poor guy to play 48 min. I use such a pattern, or a PG-SG-SF type pattern quite often, and I have no 48-min aim.
It is a fact that using a single player as Starter, Backup and Reserved (SB&R) at a single position will result (most of the times - unless injured, fouled-out, or a bug...) of 48 minutes for that player.
I will also guess that in case you will use a single player both as your SB&R at more than a single position, he will still most likely play 48 minutes.
In any case, one that asks a player to play SB&R is taking at least a chance for that.
Usually, he means it this way (48 minutes). Otherwise, he will not use this option.