the semantics of forcing the user to pass an empty value to get to the next useful parameter is the issue. "we can already pass $null there" is easy for someone who knows the API to say. Please tell me how you would explain this to someone who has never used $tip before, and potentially doesn't know what $null is
Keep in mind it's not just because you dislike very much the semantic in place that it will prevent its usage.
I undertand your concern and I agree this is far from being the best choice but the semantic is already there everywhere in mIRC (passing $null isn't just something you do in $tip), imo it's a good enough reason to keep going with it, no matter how bad you think it is.
Your example was an extreme, you don't start using the scripting language with $tip but I wouldn't help someone with $tip knowing he doesn't know what $null is, pretty simple.