Some developing environment/languages save the source code you write in a binary/propietary format that you cannot see or edit with a generic text editor. I'm not talking about compiled code, but the source code.

An example could be PowerBuilder and Oracle Forms.

It's ok you use proprietary technology if you want, but not being able to open the source code you wrote, in a simple editor, if only to read it, seems like a very strict form of vendor lock-in.

Also this prevents you from using text-based version controls that can show you the difference between two versions in a line-by-line base.

If the code is plain text, you don't need a license in order to just open it, see it and learn from it.

Should it be a golden rule to avoid vendor lock-in to avoid technologies that save your source code to anything but plain text files ?