passing $null isn't just something you do in $tip
But it is. There are actually very few places in the language where $null is ever used as a parameter unless you're doing something very weird. It's mostly used in if conditions, but not command arguments. This is why it's so confusing and unintuitive.
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.
Please don't assume to know how scripters start out. People start scripting for whatever their needs may be. Scripting something that pops up custom information triggered by an event is a perfectly valid "first script" project. Note that I did not say "never programmed before"; there are plenty of people who can program and want to do X in mIRC but aren't comfortable with the API. It would not be easy to explain this to new or knowledgable programmers.
I would say that + should be used as a prefix in iconfn to indicate that the parameter is a set of switches. There is precedent for prefixing switches with +, albeit in /commands, but it would be easier to explain.