Let me be clear, when I say “single vendor” I’m talking about being “single vendor” in what you work on, not necessarily what you install (although one basically forces the other) and what I really mean is multilingual.  I’ll explain after a brief history of why I am the way I am.
I’m idealistic but I’m also realistic.  I generally propose solutions that I think are best even if it is non-standard or out of current comfort level along with an alternative or two.  I’ve never had the opportunity to work in a single vendor shop.  As an art student in the 1990s I had been gaming over a 10Base-2 network that my roommate and I had built and realized that building that network was more fun than playing Doom over IPX.  I wanted to get into networking so badly that I worked on almost anything that presented itself.  In the early days I would take on all of the projects that no one wanted, often on things I had never seen before, learning them on the fly just so I could get my hands on more things to learn from.  This was pretty easy to get away with in the 1990s since networking wasn’t considered as much of a utility and folks willing to work on these kinds of things were few and far between.  It gave me an environment very conducive to learning every aspect of a protocol or platform.  This actually made me more versatile and essentially forced me to learn standards based networking.  It removed the reliance on proprietary technologies and forced me to really know the protocols or at the very least know where to find the info.  It taught me early on that memorizing was not the best way for me and that I had to know were to look instead of just knowing.   Doing work like this taught me how to study better than college had.
I’ve made my insignificant mark in the world of networking by being able to adapt to whatever I had to work on, admittedly sometimes begrudgingly.   Just like almost everyone else, my base was in Cisco IOS but I always had a Cisco and a something else to work on or install.  Be that an Ascend, Netopia, OpenRoute, Livingston/Lucent, Xylan, Juniper, Redback, Force10, Extreme, Fujitsu, Foundry, Nortel…..this list can go on and on.  I found that being able to adapt and work on whatever was there opened my mind to different and often more simple and supportable ways of doing things and freed me from the tunnel vision that can sometimes accompany the marketing speak.
In the networking world, Cisco has the lions share of the market, no question.  Cisco has been a company of acquisitions and one could probably successfully argue that working on CatOS, IOS, NXOS and IOS-XR is technically “multi-vendor” by the spirit of the law.  That’s actually a valid point, however, vendors tend to build and market proprietary solutions based around their products.  They want you to buy their widget, their cog, their $whatever, because they all work together.
Although I’m sure it is close to true, I tire of the old adage “no one ever got fired for buying cisco”.
Now, before you go and get your grumpy old man pants on and start flashing your Cisco tattoo, let me better explain my logic. Network operating systems are not unlike programming languages. I don’t think anyone will argue against that too passionately.  Programming languages are generally very different in structure from language to language and are processed in the brain much like a spoken or written language.  Studies have shown that being multilingual actually makes the brain more nimble, quicker, better able to deal with ambiguities, resolve conflicts and even resist Alzheimer’s disease and other forms of dementia longer. Being able to context switch keeps your mind sharp.
In addition, utilizing hardware and software from different vendors forces standards based networking and reduces what I consider a crutch of relying on proprietary protocols that stifle the ability to migrate to different vendors.
I have run into the excuse of “we already have institutional knowledge of x” all throughout my career, and while I certainly understand the viewpoint, I find this to be a surmountable excuse.  If there exists a product or solution that is better suited for the task at hand, brand should be irrelevant.  There are enough valid, cost effective and [arguably] technically superior options out there that brand loyalty should take a far, far back seat.  Learning is our job and we need to be willing and able to adapt to thrive.  I’m not saying that comfort level and familiarity should not be a factor, not at all.  Supportability is paramount.  However, that should be a factor, not the only factor.
Don’t be afraid to take risks.  Lab up new things.  You will truly never know if something is better, worse or a wash unless you give it a chance.  You may just learn something by accident.
The point I’m trying to make is that we do ourselves and potentially the consumers of the service we build a disservice when we blindly write off options because of our own personal comfort level.