How relevant is RPGmaker for actual development?

RPG Maker is pretty capable as far as standard RPG development goes. If you are looking to make something other than a JRPG-style title, look elsewhere. But if putting together a JRPG is your aim, than RPG Maker has you covered. The engine has enough flexibility to allow you to push it in some interesting directions. But it isn't designed for flexibility, so you aren't going to be re-tooling it for making a platformer or an action game. It's a blank slate in terms of content, but not in terms of structure.

I would also strongly advise trying the latest version of RPG Maker, RPG Maker MV. RPG Maker VX Ace was the previous version. It is decently capable, but it lacks certain features that just about anyone would want from a modern game engine. RPG Maker MV has all of the target platform options you would normally want, with exporting to Windows, Mac OSX, Linux, iOS, Android, and even HTML5/JavaScript/Web.

Customization for MV is done through the use of JavaScript. They switched over to using javascript as the core scripting language, and it seems to work fairly well. Be prepared to pick up some javascript if you're looking for more extensive customization. MV does have a pretty nice plug-in system, so it is easy to create your own plugins or use other people's created plug-ins.

If your aim is to make a JRPG specifically, I would recommend RPG Maker over Gamemaker. It is much more focused, with all of the structures and options you would need ready to go right out of the gate. It even has baked-in default graphics, so you can jump right to content creation from the word go. In Gamemaker you would have to spend considerably more time constructing RPG-specific tools and systems. I also recommend acquiring RPG Maker from Steam, as it is fairly common for it to go on sale there. I acquired a copy of RPG Maker MV for less than $25 on sale.