No evacuation and no Thessaloniki would be extremely bad for Serbia, which would be devastated by the CP occupation even worse than IOTL...but it won't be a major benefit for the Central Powers war effort and, depending on how things proceed, it might not be a benefit to the Central Powers at all.
It frees up troops for other fronts - some CP troops, but also over a dozen French and British (and Italian) divisions. According to some estimates, the lack of a Macedonian front would even actually benefit the Entente - I'm not 100% sure, but it's quite possible. Especially as Bulgaria will almost certainly declare it has done its part of the job and spend the rest of the war sitting on the sidelines.