If scripting is just a way of opening the power of Plus! up to the more advanced users then why not just make it open source? (possibly to a small group of people - there aren't that many who can actually do C++) This would also give you people submitting patches and enhancements with code (alright, that's not really true, but it is possible
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Otherwise I think you run the risk of completely duplicating the existing MSN object model, but in a harder to use way, Plus! should just be an MSN addon, not a programming environment.
There are a few advantages to scripting, but not enough to outweigh the pain and number of complaints/questions about it.
The other factor is stability, I use Plus! on a server that runs 10 weeks at a time, if someone contributes a script that starts to leak resources then resource managers will just see Plus! as the culprit.
The final point is virus's, when someone writes a Plus! virus, people will blame you (wrongly, but hey, thats life). Not everyone who uses MSN seems to be nice (a worryingly large number seem to be pure evil...
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When I said you would have to like COM I just meant that COM in C++ never made sense to me (I use VB if I need COM) but you would need lots of it, for VBScript and JScript it is simple. All a VB/Jscript ever does is create an manipulate COM objects - that's all they can do, but that's all a VB program does also.