See, Caterham, Marussia and HRT often (ahem, always) qualify on the back of the grid anyway. And the penalty of using too many engines, is a grid penalty. So for them the penalty is so little, why don't they just supercharge their engines, so they last only one race? Maybe then they could get closer to the points from the back of the grid.
Among other things because engines are very expensive.
If McLaren has to "face" soon the payment of the Mercedes engines, and that's something they are taking into consideration, for example, while negotiating the driver's new contracts, imagine how much it would mean for HRT to pay 3 times what they pay now for engines.
And also because, even if they are not fighting for points, they are fighting for positions in the constructors championship. Making your life hard at the beginning of every race is not necessarily the best way to win the Catherham/Marussia/HRT championship. It would be arguable if a newer engine compensates beginning from further back or not.