Just edit often used files in the main window.

If I want to see/edit a file, just have it full on in the main window,
there is really no need to do it in a split window (or, god forbid, like
before that: in a tab)... just edit and let me decide what to do next
with the buffer.

In the future, it might be a nice addition to handle one of the
often-used files like this scratch plugin (albeit it more persistent
then): https://github.com/mtth/scratch.vim
(In that plugin, the scratch buffer closes automatically in certain
cases, which seems to be a handy feature... I don't like the
non-persistent aspect of the plugin though)