Defence + is surely necessary. The point is that these days so much malware gets produced non of the av makers can keep up with that. Even with the help of heuristics and generic signatures. So, other solutions are needed. Behaviour blocking is a strategy but with a classical HIPS like D+ you will catch more undetected malware.
In a recent article on ZD Net it was reported that this spring new malwares saw the light that were not detected anymore by the generic signatures of the av companies across the board:
http://news.zdnet.com/2100-9595_22-320602.html .
I hope that shed some light on why you need D+.