Being able to enforce  a "no alt accounts" rule is a separate issue from having the rule.  There will always be ways to fool the enforcement. 

The absence of the "no alt accounts" rule rewards dishonesty.  If there is no penalty for creating an alt account, there is no disincentive to having sock puppets that bump your threads, shill your auctions and anonymously attack viewpoints you don't agree with.

I have not yet seen anyone present a good reason for having an alt account.  So far, every alt account I have become aware of has existed for nefarious purpose.