I'm not sure what you mean by standard, because I see nothing about the standard usage of x, s and m in perlre.
You changed from "well-known semantics" to "well-known default semantics". My point is that they're not the same. The semantics of ., ^ and $ are not well-known, since they are context-sensitive. The defaults are overriden regularly — more often than not, in my experience — so relying on the default semantics is no good. If you wanted well known semantics in your code, without loss of functionality, it would have to be done through the enforcement of xms.
The problem is that the world on which xms can be enforced is limited, so the "well-known semantics" would only be well-known within that world. However, that's more consistency in semantics than one would enjoy otherwise.