I would definately support that idea for extended /echo RGB color chart.
I could see how compromising the 0-15 color range could bug people who were using older versions of mIRC, the code might show up on their screen, and be non-interpretable to thier current version?
As a stepping stone, enhancing local colors would be great. I think it would catch the eye of alot of users who aren't into using dll's as a means of theme enhancement. Dll's are nice but there is a sense of satisfaction that comes along with knowing that the scheme is a result of ones own fabrication rather than whatever result the dll produces. Not to mention that a dll, is generally for advanced users, ...people who arent sure how they work have a hard time taking the step to learn. Especially when they dont really know what its doing, you just get what you get.
As to where giving them more colors to make code the way they already know how, might make things more interesting for level 1 or otherwise starter writers. In turn, promoting mIRC.