From an administrative viewpoint the "one stop shop" is attractive in terms of licensing, updating and the rest. However, it is not so good from a security angle as it presents a single target.
True, multiple vendors to some extent could increase you defense in depth.

BUT...

Also consider that multiple solution from a single vendor will (should) be made to work together so will not introduce the same risk of incmpatability that the best of breed options might.

And every different piece of software you introduce into a network carries additional risks just by it's presence. The more stuff you have, the greater the risk of a vulnerability from that stuff.

Nihils mention of licencing is probably the ultimate point in business for purchasing software.

If it can be bought from the same supplier, possibly under the same licencing and support, probably with a discount for taking another product, probably all products are integrated and guarenteed to work together.
All of that probably outweighs the benefits from defense in depth.