Why does nmap fingerprint the application by using the standard probes

[alan donald <netquantum () yahoo com>]

I was looking at a number of scanners and found that
nmap sends out generic probes and notes their
response.
Why is nmap not sending probes very specific to that
application. That would give a higher level of surety
as maybe in the case of http scanners or smtp
scanners. 
Was just wondering why this wasnt done and the current
approach was chosen. Any comments? Is this a
performance issue or some scalability issue.
Therefore using this aproach can we totally rely on
nmap to tell us the application version each time or not?

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