It doesn't make a whole lot sense in terms of cost-cutting, because it has little, if any, effect on development, which is far and away more expensive than manufacturing.
Since not a single competitive team has managed to avoid penalties, the limit is arguably too low, and it's scheduled to be reduced even further by 2019...
grandprix.com, May 25, 2016 wrote:At the time, the FIA said cost reduction will be possible through a "progressive reduction of the number of power unit elements per driver per season".
Currently, drivers can use five engines per season, but Germany's Auto Motor und Sport said that by 2019, that number will drop dramatically to just two, irrespective of the size of the race calendar.
When asked if that will even be possible for Mercedes, Ferrari, Renault and Honda to achieve, FIA race director Charlie Whiting said: “
I am confident they will succeed, because the manufacturers themselves are confident and agreed to these numbers.”
(Incidentally, justifications don't get any weaker than that.)
But, because the regulations are a patchwork of bad ideas, half ideas, and aborted ideas, all ostensibly held together by good intentions, I don't think anything about the power units will ever be logical.