Of course Spam is a perjorative term, so when considering the issue it is worth keeping that in mind. Add to this that there may be very valid reasons - or indeed limited options in the Codex/Army Book - that makes taking identical units the logical decision.
Essentially what the Spam list does it to pose this question. "You've killed the best unit I have .... how are you going to kill these three identical units?"
Which feeds into a related arguement of how much is the skill of the game related to tactics, and how much to list building.
The tactical conundrum for anyone considering this issue is that if you are playing in a spam list enviroment, then the tendancy is to build lists to deal with the Spam unit - which in turn leads to a Spam list.
The drawback of this equation is that it becomes the easy default position and works against experimentation - both tactically, and with formations and terrain placement - and perhaps the critical element in the game, movement.