Yeah, but...how can you give a good reason for starting with presuppositional apologetics without contradicting your own premise? By definition, it would appear to be impossible. Using it to witness to an unbeliever presupposes that the unbeliever first accepts the scripture, which almost assumes that the unbeliever is a believer. Sometimes God reaches people with scripture, sometimes with reason, sometimes with tragedy, sometimes relationships, and even with conflict. Fullness of the knowledge of Christ comes through scripture, but it need not start there.
When dealing with an unbeliever, let's say an atheist, I am fully aware that he/she doesn't share my view of the Scriptures. But I don't lay down my sword or toss it away...I use it.
This Thanksgiving holiday I'm thankful that you are a Presuppostionalist =)