There is always room to add diversity as long as a cost is associated with it. And I don't just mean resoucres. Time is just as important of a resource, with teching playing a big part.
The Protoss can be diverse, but, like the other races, they have to branch down a specific tech tree to obtain them. This way you have more overall options but you are still tailoring yourself through your decisions.
In the case of defenses, they may need to tech a little to get better defenses, but they should still be there.
I think diversity is ALWAYS better. It just gives you more options at your disposal. But because of the costs associated with diverstiy, it keeps a balance insured.