Many users move windows around temporarily, especially when it comes to IRC, where channels are joined, parted, maximized, and minimized constantly depending on activity. Therefore, it's better to have an explicit setting to "save" the location rather than implicitly remembering locations just because they've been moved. I think that's a simple explanation, and summarizes Khaled's position on the issue.
MDI applications are intended to run maximized.
Please cite an official source for your claim. I've never heard that before in 15 years of using MDI applications in Windows. If this were true, MDI windows wouldn't have a "cascade" organization feature. MDI is simply a way to organize an application's windows hierarchically. I suppose this is all moot though, since mIRC is not getting rid of MDI any time soon.