You're right, not everyone is a Billy Graham, but that doesn't mean the Lord can't, or doesn't use them. How about Moses? He had a speech problem but look at all that was accomplished through him:
Amplified Version - Exodus 4:10 & 11 -
10 And Moses said to the Lord, O Lord, I am not eloquent or a man of words, neither before nor since You have spoken to Your servant; for I am slow of speech and have a heavy and awkward tongue.
11 And the Lord said to him, Who has made man's mouth? Or who makes the dumb, or the deaf, or the seeing, or the blind? Is it not I, the Lord?
12 Now therefore go, and I will be with your mouth and will teach you what you shall say.
Sometimes it can be through things that we consider to be crazy, or foolish, that God is able to reach a lost or hurting soul:
1 Corinthians 1:26-28
26 For ye see your calling, brethren, how that not many wise men after the flesh, not many mighty, not many noble, are called:
27 But God hath chosen the foolish things of the world to confound the wise; and God hath chosen the weak things of the world to confound the things which are mighty;
28 And base things of the world, and things which are despised, hath God chosen, yea, and things which are not, to bring to nought things that are:
29 That no flesh should glory in his presence.
Did Balaam's donkey speak? Then why should we think a human being can't be used?
How about all of these less than perfect people?
Peter was a gruff man who even cussed and denied Jesus.
Thomas was full of doubt and unbelief.
Paul persecuted Christians and was a 'know it all jerk.'
James and John were envious and argued over their position.
David committed adultery and murder.
Noah pitched a drunk.
Samson had a problem with women.
Saul sought direction/information from a witch instead of God.
Elijah ran in fear from Jezebel.
And the list of misfits and those we would consider unlikely to be used by God goes on and on. Maybe this lady is in the ministry to reach people that no one else could ever reach. Maybe she is still hasn't yet got the confidence that will come with time. Maybe she is very aware of her "problem" and despises it! But she still chooses to walk in obedience to the calling on her life regardless of how she feels... or what others may think of her. At least she is evidently willing to preach/teach the Word. Maybe she's in the position she is in simply because she is willing and has been placed there by God because someone that WE might feel is better qualified is NOT willing to be there.
I guess there is really no end to this debate because it could go on and on, with one example after another, but there has to be a stopping point - and it might as well be right here... for ME, anyway.
Lisa