This way lies madness...
In all seriousness, conditional logic would be useful, but I think some high-level design decisions need to be made about how far to go with such things.
You'll no sooner have IF then you'll want ELSE, and then SWITCH, and pretty soon you'll have a whole scripting language like MapTool. It's "powerful", but it is an ugly syntax, borne of epic feature creep.
If we're going to start programming macros in Roll20 (*and I wouldn't mind that*) then let's just start with a proper scripting language. Roll20 would only need to provide one side of the interface, and the rest could be handled by a system-specific bit of code coming in from somewhere else.
People who don't know JavaScript (or whichever) might be scared, but those are the same people who will start the inexorable march toward a piecemeal pseudo-language by asking for one control structure at a time, then turn around in 3 years and lament that the syntax is brutal and they wish they could just make it talk to JavaScript...
Can you tell this is the voice of experience speaking?