A signature that is legible or illegible should not be the issue at all.  The person's LEGAL signature should be what matters.  Most adults have a legal signature used for transacting business with banks; filing tax returns; executing wills, advanced medical directives, burial instructions and powers-of-attorney; purchasing vehicles; and virtually every other legal aspect of their lives requiring a signature.  Legibility does not enter into the equation.  Most often, a person's legal signature is highly stylized to make it difficult to forge.

If we toss out every signature in life that is illegible, I wonder how many pharmacies would fill prescriptions?  

Just look at the signatures on the Declaration of Independence.  How many signers would have their signatures tossed out as illegible or are guilty of a material ommission?