I diagnosed a few problems over the years that arose as apparent issues with a web application but that I gradually narrowed down to things like network issues, or kernel bugs, or system misconfiguration, or database issues etc. Modern stacks are very complicated and the interactions can get really messy, it is close impossible for someone who doesn't understand the whole thing to find issues that aren't neatly isolated. I perfectly know that I do not have the full qualifications of a sys-admin proper, and would not like to do a sys-admin job full time, but in those particular cases a pure sys-admin would not (and often actually could not) find those issues. As an example, I can remember many situations where the application showed different behaviour depending on which application server you hit, and typically both "pure" developers and "pure" sys-admins were having a hard time finding the issue.
Good sys-admins anyway have to learn, at least, C programming, shell scripting, and network protocols and programming, so it's should not be a big deal to add some Rails/Django/Node to their skillset. Good developers anyway have to know things about hardware, networks, protocols and so forth. You do want to have people that are specialized in one or the other area and focus on it on a day to day basis, but you also do want to have people that can understand a particular aspect of the system top to bottom when such a need occurs, and it does happen quite often.
I'm all for a tighter integration between Ops and Devs, and infrastructure-as-code can help bridge that gap, but I don't know that doing each others jobs is the solution.
At the plus side, you have quite a big deal of control over most of those variables.