Sebastian Vettel has two unused engines left to him for the remainder of the season, with six races to go, including Spa. Red Bull have two options, either take the conservative route, and make two engines last for six races, or be aggressive, and take the hit, i.e. deal with a penalty in one race as the price for using an extra engine.
The latter choice might be better. Ideally, if they took on the extra engine late in the year, when the weather might be more likely to feature rain, in which case the grid penalty might be less of a factor. If you manage tires and pit stops right, making up ten slots in the rain can be fairly easy. (Of course, if you get it wrong, it’s generally a complete disaster.)
Vettel is probably a long shot for the title now, anyway. Although, in theory, anything could happen, he has three drivers ahead of him in the points, Button, Barrichello and Webber, in that order. It might be that he’ll just be racing for wins and manufacturer’s points for the rest of the season, which, again, might indicate that the less conservative choice above would be the better option.