It is in fact not merely necessary because the OS will overcommit. Even without overcommit, you hit the same problem, except you have no way to decide how to handle it---the application requesting the allocation suffers.

I guess my entire point with respect to defaults is that for anyone who knows what they're doing and the defaults do not work for them, they already have a huge pile of systems design and architecture work to do, and the mechanics of changing this OOM policy for that is a trivial change.

For people who do not know what they're doing and want the OS to do the hard stuff for them, there is no sane default. There are tradeoffs that will make large numbers of people unhappy. It is a no-win situation.