I think the case in point to your argument is this:
All of the "buggy" parts of mIRC under wine are known wine bugs (or features that are not yet implemented). Wine is an open-source product, ie., you are completely free to work on fixing these bugs. And yet, the bugs remain, and people (you?) keep complaining about how "buggy" mIRC is under wine. The bugs have all been around for many years now, and nobody's tackled them. Do you see where I'm going with this?
If you're so interested in contributing to making mIRC work under other platforms, contribute to wine. The fact that nobody has done so shows that: a) it's a lot easier to talk than to do, and b) multi-platform support is not easy, no matter how it's done, and most importantly c) "open source" didn't yet solve the problem in the first place.
Wine is what you should be looking at; not only is it already open source, but it's the source of the bugs anyway. Even if mIRC was open sourced, it wouldn't help you w.r.t Wine's issues, because the bugs are on Wine's end. Patching mIRC will cause more problems in the future when (if?) these bugs are fixed in Wine, so Khaled usually refrains from implementing Wine-specific workarounds. An open-sourced mIRC would have the same policy (I'd hope), so OSS isn't the answer for Wine support.
The real problem with mIRC under wine is that everyone gets frustrated with the bugs, goes onto the wrong forums (mIRC's) and complains about how mIRC doesn't work well under wine-- instead of going to the right place (wine's bug tracker), reporting a bug and seeing it through. If people directed their energy towards practical solutions, we'd be in a much better place.
Finally, to put things into perspective, I should point out that I use mIRC under Wine quite a lot. It's not nearly as buggy as people seem to imply. It has its quirks, and some things glitch about, but it's stable and completely usable. So, I don't even think mIRC is in any dire need of better multi-platform support. It could certainly be improved, but it's hardly a reason to go as far as to call for the open sourcing of an IRC client.
