Technically, you cannot check the run-time usage of your variables concerning strings/numbers in compile time (or before, using 'lint' or somesuch) with a garrantee of success, because you don't have any idea of what data your program will read in from external sources. (Keyboard, files, databases, etc.)

So even if your code compiles without any 'numeric/string warnings' it may still fail in runtime when handling unforeseen data.

I guess the reasoning goes something like: "Since we cannot garrantee a successful verification in compile time but the programmer must anyway do it in run-time when it's important, then le'ts be consistent and let the programmer do the verification whenever s/he thinks it is necessary."

Personally, I tend to agree.