When does it become spam?

This happens a lot more in 40K than it does in Fantasy, but when does army composition become spam?  If you have 3 10-man Grey Hunter units in a Rhino, is that considered spam?  What about 3x Baal Preds in your army?  What about taking 3x 10-man units of Assault Space Marines geared out in the same way?

What exactly qualifies as spam?

At what number does it qualify as spammy?

Do they have to be the same loadout?

Is 40K a repetitive game with redundant units?  Without redundancy, it appears that lists are less effective because there's a higher chance they won't impact the game if they're destroyed.  The argument for most people when they ask the old-aged question of:  How many Land Raiders should you take?  Most people reply with 2.  Why 2?  Because if one goes down, you have another.  At the same time, these players are also the same players that bitch about 3-4 Razorbacks with TLLC sitting across the field from them.

Personally, I don't think spam is necessarily a bad thing in a game like 40K.  In fact, I don't think the game is designed to promote different units on the battlefield.  Is this a problem with internal book balance (useless units vs. non), or is this just player mentality?  Something to think about.