1. Linux is open source and free as in speech, not as in beer. I am not prepared to say that it will cost any less to set up a Linux vs Windows based systems.  However, Joe the Plumber will have a better idea of what he is paying for, services much like the services he provides his customers.
2. Joe may be small but to compete he needs to to be responsive. Joe’s Linux Homeserver has been set up so information is accessible on the road, in fact he has an infrastructure solution comparable with the Big Boys.  It is unlikely that Joe the plumber could afford all the software licenses required to set up a system in the Windows World.
3. Security is a concern for Joe the plumber and his inbox is always full of spam. Linux is open to attack like all software solutions however it is allot more secure (by Default) than Windows.
4. Joe the plumber wanted to implement a small website in order to increase his visibility in the local market.  Services for the Windows (.NET/ASP) solutions appear to be significantly more expensive than the Open source solutions. Now that is just weird particularly as Opensource is good enough for some of the biggest sites in the World (Amazon, Google)

Conclusion, for a small business who see technology and connectivity as a competitive benefit, Linux is a firm base on which to base systems.