I asked Zach Holman at one of his tech talks a couple years ago and he replied (paraphrasing): "Yes read/write deploy keys are not ideal. That's why we don't ourselves use deploy keys at GitHub, and in fact we regret that the feature exists." I think he used an expression like "deploy keys are a red-headed stepchild".
I like to pride myself on never having used a clumsy phrase like that. That said, machine users were en vogue at GitHub, although maybe that's changing with more consideration being given to deploy keys nowadays.
Sorry, I believe you used a more palatable expression than that, but having a similar meaning.
Edit: and to paint a more complete picture, you pointed out in your answer that many folks were using deploy keys to promote from a development branch to the master branch. Some commenters in this thread may, like myself, just not have thought to use deploy keys for this purpose, or that others would want to.
