That depends whether you think the point of libertarianism is preferring market solutions to economic problems, or the protection of pure property rights.
The latter is perhaps more ideologically pure, but I think the former tends to lead to much more sensible arguments.
Well I realise I'm biased, but the libertarians I respect tend more towards "market freedom as the null hypothesis" rather than the ideological extremes that lead to rhetorical nonsense like "tax is theft" and "property rights can be derived from non-aggression", or my personal favourite "violent coercion is bad (unless it's used to enforce property rights, that doesn't count)".
"...what aggression is depends on what our (property) rights are...One cannot identify an act of aggression without implicitly assigning a corresponding property right to the victim."
For example, driving a car can be aggression, depending on whether I have property rights to that car or not.
I'd need to see the context, but this seems even more ridiculous than calling tax theft. Taxation just moves things around, it doesn't destroy anything. (It might cause destruction due to inefficiency, but that's a separate argument.)
>Property rights aren't a consequence of non-aggression, so much as non-aggression is scoped by the extent of property rights.
Well this is more consistent, but I still consider that a fairly ridiculous definition of aggression. Most people would consider aggression to mean actual violence (or threats thereof) and nothing else. Using resources outside the constraints of society's property system (i.e. theft) might be considered immoral, but I wouldn't call that aggression.
Defining aggression to include theft just seems like a rhetorical trick to avoid explaining the immorality of theft itself (if it's called aggression it must be bad right?)
The latter is perhaps more ideologically pure, but I think the former tends to lead to much more sensible arguments.