My question is why do we even need an extra 2 umpires during the playoffs? In general when the playoffs start at any level, extra umpires are added under the assumption that more is better when in reality is is not. When you do things the same way in the case of MLB umpires for 162 games it can be a problem to change rotations and coverage etc... IMO I think that umpires are also added kind of as a reward to those umpires for their good work during the year but again it comes at a price because it very much changes the way you umpire throughout the whole year. IMO Holbrook making a call like he did last night which was in a position very much out of his norm, was a factor in why the bad call was made.
If a 3rd base or 1st base umpire cannot make the same calls that the umpires on the lines are suddenly hired to make especially in this era of replay then there is a problem.