Subject: Why is filling the valence shell so important?

As a high school chemistry teacher, every book we teach from explains 
bonding as a phenonemon that occurs so that atoms can fill their 
valence shells. I am really questioning this now. Why? For a few 
reasons. Let us look at sodium and chlorine forming sodium 
chloride. (1) Chlorine when it gains an electron actually increases 
its size, increasing the potential energy between the electrons and 
the protons! Bad. (2) Sodium, we tell the students wants to lose an 
electron so to have an electronic configuation like Neon, but it 
requires energy to take this electron away! Bad. Sodium and Chloride 
combining is driven by the ionic bonding step, not the formation of 
the ions. Is it that sodium can give an electron fairly easy, and is 
left with a full valence as a consequence only and chlorine can grab 
an electron because it has a good electron affinity, and it has a 
place for it in the valence shell. Both of these do not necessarily 
mean that having a filled valence shell is important. I am now 
wondering why we tell the students that filling the valence shell is 
the driving force for bonding. What energy is minimized when the 
valence shell is filled for simple atoms (periods 1-4 let's say)? Is 
it some sort of quantum energy? If it is I would like to know fairly 
simply and explicitly.

Re: Why is filling the valence shell so important?