We go further than that, though: we think you should be multi-platform. For example, if you have lots of Linux boxes, we really think you should have at least one other flavor of boxes (e.g. SPARC Solaris boxes), and you should manage them as a consistent whole. Why? Well, there are three basic reasons.

The first main reason for "multi-platformism" is to avoid vendor lock-in. If all you have is Solaris boxes, you will end up with all sorts of Sun- and Solaris-only assumptions wired into your system -- even if you try hard to be good and avoid that.
Why is vendor lock-in bad? Because, obviously, they can hold you to ransom (and, smilingly and oh so politely, they probably will). Another reason is that vendors go through bad patches and, if you have no alternatives, you're stuck. For example, in the late '90s, Sun was unable to deliver a SPARC CPU that answered to the name "fast". If you're locked in and speed is your thing, all you can do is wait and hope for the best.
The second -- and much bigger --- reason for "multi-platformism" is that it makes for great sysadmin discipline, and that will pay off in pretty short order. If, for any solution, you've got to make sure it works for at least two different platforms, then you're probably going to think the whole thing through much better. That's good.
The third reason is that the previous two combine towards being good at change (a key ARK idea).
The fourth reason is that you're going to be multi-platform anyway, at least occasionally.
Even if you are a resolutely single-platform shop, you will need to upgrade your OS, say, once every five years. Unless you are of the `flip the switch and pray' school, you will be multi-platform during the time you are testing and switching over to the new OS version.

We don't think that the multi-platform thing should be carried to extremes; for example, I once read of a site with 14 distinct platforms! That doesn't require discipline, it requires gratuitous pain.

Somewhere between two and four distinct platforms is about right. If one of those is a properly 64-bit system (e.g. Alphas), that will probably stand you in good stead (work all the 64-bit bugs out of your thinking/system).

(NB: you can be a full ARK enthusiast even if you think we're bonkers on this multiplatform-ism thing...)