The problem with the MDX way of doing is the that the day he would of wanted to add container controls like a rebar, tab, panel, etc it would of shoewed the limitation as you can't put mIRC regular controls on these, mIRC "loses" them because they don't touch the parent dialog anymore. That's why my first attempt at DCX in 2004 failed because I couldn't realize that fact and tried all sorts of nasty workarounds that never came through.
The new DCX is the only way to approach the problem in a reasonable manner and I think I greatly simplified the usage with making things conform to a generic interface identical to mIRC's /did, $did, /dialog and $dialog.
