Can Semantic Values be Neutralized?

The argument of those who claim that Greek is tenseless basically says that because there are times where “presents” are in past and future contexts and then there are times where “aorists” are used in present and future contexts.

My question is this: Why cannot semantic values be neutralized by pragmatics?

Campbell is the most clear on this in his glossary (under “P” and “S”) for Basics of Verbal Aspect:

Pragmatics: The cancelable outcome of all textual/lexical/deictic factors in combination. What a verb ends up “doing” in the context.

Semantics: The uncancelable core value/s of a verb form. With respect to verbs (verbal semantics, grammatical semantics), the semantic values are aspect and remoteness or proximity (or tense).

I’m not picking on Campbell specifically, its just that as far as I have read, makes this distinction the most clear. The way Porter formulates his arguments, I can only assume that he believes the same thing.

But why is this necessary? Why can’t the pragmatic outcome of factors in a phrase/clause/discourse result in the neutralization of a semantic value – whether tense or aspect?

I would especially be interested what those of you who hold such a view of the Greek verb think of this question.