Traditional router solutions can often fence you in, either in terms of support for new technologies ('No, you have to buy chassis XYZ to use interface B') or in terms of software and features ('This unit will only do X; buy our fancy Y unit if you need Y.') In theory, you should be able to sit down and design a network, work with the vendor to map out a solution, implement it and forget it. However, this isn't the reality for most environments. If things aren't moving and changing constantly, either your business isn't growing, or you're ignoring the advances in technology going on around you. The former can place extraordinary demands on a network admin, especially one who has to work within a budget. The latter is simply the name of the game. If you're upset with the idea of technological change, perhaps network administration isn't a good fit for you. What other benefits do Linux routers deliver?
Routers also need to be easily monitored and maintained. This is an area where I think Linux has a great advantage over other, more traditional router platforms. Just as I know best what network components I need in place to help my company succeed, I know best what type of monitoring and notification system works for my support staff. Some folks need to be paged, others want e-mail and still others want a Web site dashboard with pretty graphs. But perhaps I can't afford something [that offers that functionality]
In a word, flexibility. Linux, by the very nature of its openness, keeps pace with new and evolving technologies, so you're picking a platform that will be there when IPv6 running over 48-terabit fiber is the standard desktop load. But, at the same time, Linux is not only used for experimental development. Routers, above all, have to be stable under constant load. The fact that Linux is widely deployed as a workhorse network and server platform means that the code has been exercised in countless different scenarios over countless hours of production uptime. Speaking of affordability, does Linux's reputation for thriftiness carry over to routers?
Of course, there's always the question of cost. Linux is free, in multiple senses of the word, but the savings go beyond that. Running your router on a commodity platform like a good server PC means that you're getting the absolute best ratio of technology to price available because -- in many cases -- you're moving your routing function off a piece of proprietary hardware onto the most ubiquitous of computer hardware offerings, the 1U rack-mounted server. But even if this isn't your target platform, you still have some 'upgrade protection,' as it were, because you have free reign to choose your hardware and the support you use for that hardware. The total savings may dwarf the initial 'Linux-is-free' discount.
FOR MORE INFORMATION:
Linux networking tips: Practical Guide to Red Hat Linux, Chapter 9