Lunar Magic, probably the most popular ROM editor to date is closed source and for Windows. Mac nor Windows users could not use it for a long time, and the creator himself had no notes, just the editor. Years later a site called SMWCentral was made, where their goal is to gather as much as information from SMW as possible. There are several public documents available now, and just about anyone can edit the game, regardless of their operating system. If the man had released the source in the first place, we would've had documents of all the things it can edit and probably even ports.
MarCas, the only Super Mario Land 2 editor is closed-source, and remains so (I talked with the guy and apparently the code is so messy that he is even ashamed of it). Fortunately, the man was kind enough to teach me all he knew of the game, which inspired me to start my
own document archive. It is a combination of his and my personal research. Now anyone can edit the game.
NSMB Editor, made by our very own Treeki, is open source. Treeki has already made several documents of the game mechanics, and pretty much anyone can edit it now.
My point is, everyone is saved from a lot of trouble if a source is released.