There are obvious advantages from getting all of your solutions from one point; interoperability, consistent interface, easier to support and troubleshoot, licensing and cost benefits.
Centralization is the keyword here... when things need to get done, you have one number to call, one representative from one vendor to get the wheels movin.

and also validate that if the right combination of attack packets comes along, the whole house of cards won't come tumbling down?
I can't really see a situation where a one vendor network can be crippled from the same vector, perhaps you had one in mind? Let's think of this though, if 20 devices fail, and you need to get back up and running, do you want spend time with 20 different vendor reps, or would you rather start limiting your risk with one phone call? Response time is a benefit with one vendor, they pay attention to you when you're a valued custy


edit:

This is kind of related. I had a conversation with someone who felt that it would be a better idea to have unique firewalls everywhere there should be one. The reasoning was that the same exploit couldn't be used on all the firewalls, limiting your risk.

However, this is a bad idea because it increases risk, you now have a new potential vector for every new firewall you put up. If an attacker wants in, they have a wide array of choices and just need to wait for a new vuln for any of those firewalls to pop up.