The general concept of "substitution" can violate ethical and/or legal rules.
(1)A student has a friend take his tests for him
(2)Harrah's is running a casino promo, if you play in x number of their casinos nationwide you get y bonus points; discussed in another forum on ft, how people would swap their casino cards and play for other people.
(3)Taking a defensive driving course on-line (to get insurance and/or point reduction on your driving record), leaves the system open to substitutes taking the course for you.
(4)Having substitutes take SAT (Scholastic Aptitude Test) for them, to get higher scores and land places in better colleges. [This led to a big scandal on LI recently.]
(1)-(4) and the theme of this thread, are rule violations, but you add:
(5)A train motorman, bus driver (even a pilot) has a "competent" friend fill in for him--now the substitution can lead to serious safety issues (where lives are at stake).
Bottom line, IMHO, even if you can get away with this--don't.