Renderers need to deallocate memory all the time. For example, they often need to store per-frame data that gets dropped when the frame is over, debug strings, track temporary access to texture/buffer data, manage command buffers, etc. They also have to track resources for lots of data types external to the program (such as GPU data) which often forces them to work with external allocators, as well as having to synchronize stuff like resource destruction using GPU-CPU fences. Moreover, most resources are either immutable or can be accessed optimally when they are immutable, so you often end up benefiting significantly from being able to destroy and reclaim the memory of stuff you're no longer using rather than trying to update in place.

Even if you try to only allocate one buffer (for example) to split up and use for all your logical vertex buffers, you still have to manage memory within that buffer, which ends up leading to pretty much the same kinds of complex reasoning you need for ordinary dynamic allocation. Failure to do this properly results in resource leaks on both the GPU and CPU side which can be really nasty.

Of course, this all depends on the complexity of your renderer. For simple usecases you might be able to get away with no deallocation until exit. But they'd have to be really simple.