A slower background transcode usually doesn't matter, but a faster transcode that stops important processes running in the meantime might. This is usually fixable with effort, but sometimes it's nice to not have to configure everything to the nth degree.
I don't really buy it. The idea that somehow getting one less core but faster per core speeds per pricing bracket makes any difference in this imagined problem.
There are many different configurations of vps available with different numbers of cores, if you are picking the vps configuration specifically to have more cores than some transcoding software uses by default to avoid configuring a thread limit for that software then you are still configuring things to the nth degree just at the objectively wrong level of abstraction.
You don't have to buy anything, however I ran into this exact issue two days ago. YMMV