Just as you may get additional security from using a number of different systems to provide depth of defense, you may also get additional security from using a number of different types of systems. If all of your systems are the same, somebody who knows how to break into one of them probably knows how to break into all of them.
The idea behind diversity of defense is that using security systems from different vendors may reduce the chances of a common bug or configuration error that compromises them all. There is a tradeoff in terms of complexity and cost, however. Procuring and installing multiple different systems is going to be more difficult, take longer, and be more expensive than procuring and installing a single system (or even several identical systems). You're going to have to buy the multiple systems (at reduced discounts from each vendor, because you're buying less from them) and multiple support contracts to cover them. It's also going to take additional time and effort for your staff to learn how to deal with these different systems.
Beware of illusionary diversity. Simply using different vendors' UNIX systems probably won't buy you diversity, because most UNIX systems are derived from either the BSD or System V source code. Further, most common UNIX networking applications (such as Sendmail, telnet/telnetd, ftp/ftpd, and so on), are derived from the BSD sources, regardless of whether they're on a BSD- or System V-based platform. There were any number of bugs and security problems in the original releases that were propagated into most of the various vendor-specific versions of these operating systems; many vendor-specific versions of UNIX still have bugs and security problems that were first discovered years ago in other versions from other vendors, and have not yet been fixed.
Also beware that diverse systems configured by the same person (or group of people) may share common problems if the problems stem from conceptual rather than technological roots. If the problem is a misunderstanding about how a particular protocol works, for example, your diverse systems may all be configured incorrectly in the same way according to that misunderstanding.
Although many sites acknowledge that using multiple types of systems could potentially increase their security, they often conclude that diversity of defense is more trouble than it's worth, and that the potential gains and security improvements aren't worth the costs. We don't dispute this; each site needs to make its own evaluation and decision concerning this issue.