Stats stack regardless of what you have equipped. So it's ideal to collect all of them as you can. This is what you get from having them all minus the time limited ones (ST and XGNW ones):



For reference:

A level 7 life maga is +384.

A level 7 attack maga is +258.

A level 7 defense maga is +169.

A level 7 ninjutsu maga is +258.

A level 7 resistance maga is +169.

A level 10 init rune is +370 and a level 8 is +210.

So it's like having multiple additional magas and runes on your characters.