PHP, Python or Perl without all their great additional modules are pretty much the same as well. Though they're all used for different purposes. But all of the have modules as well that allow you to program a game in them.
The main difference is that BlitzMAX doesn't really have any libraries that let you code anything BUT games in it, which is probably why most people see it as a hobbist/game language. No persistence framework, no regular expressions, no XML support, no multithreading, no proper unit test framework, no deployment tools, and so on.Try writing a distributed webservice using BlitzMAX. Sure you can do it, but it would be more a matter of "proving FlameDuck wrong" than "practical application", and doing it in C# would be several orders of magnitude easier, and given the lack of multithreading in BlitzMAX, it would also perform much, much better.
it can be used as a general purpose one because modules can be added, changed or removed for the more advanced use.
Sure. But if you're going to be writing a ton of boilerplate code to get all your usual C++ libraries working in BlitzMAX, there's a certain point where you might as well write the whole thing in C++ anyway, or just skip right to C#.