If that were the end of the story, though, this blog post would be pretty short. So let's look at what material advantages there might be in security monitoring, besides just (hopefully) catching attackers before they do too much (more) damage.
As I've written before, good security monitoring can tell you more about your organization than just how many nmap probes your firewall has blocked. (By the way, I don't consider that number to be at all interesting. Basing your metrics on how many packets your firewall has automatically blocked and calling them "security events" is like counting how many "water events" your roof has handled during the last rainstorm.)
Two areas in which security monitoring can help the business are in performance measurement and data flows. Performance measurement doesn't just mean the load on the server or the network bandwidth saturation. It can also mean the latency on database queries, which will almost certainly affect your application performance. It can refer to how quickly you can make configuration changes, how consistently they're done, and how long they stay configured that way. A lot of operational efficiency metrics are hidden in those logs, along with troubleshooting data. (Oh, the SSL certificate expired! That explains all the failed connections from one server to another...)
Data flows are the lifeblood of your business, and if you don't believe that, then try tripping over a network or power cable sometime. But it doesn't stop with availability of data: Many organizations don't really know who is accessing what data and why. Anyone who has tried a server migration will find this out very quickly, when other departments show up at the planning meetings to slow down the project. Knowing your highest-use data will help you understand its value; it may also tell you which business operations cross disciplines, which ones need optimization (because they're processing redundant data, for example), and where you might have opportunities that you hadn't thought about.
Business intelligence is a thing these days, and CEOs do like to hear about that. Operational efficiency is something that everyone can get behind. If you can demonstrate that security monitoring contributes uniquely to either or both of these, then you may just get permission to pay more for that fancy, new SIEM. Helping the business make more money is the next best thing to making it yourself. The outlook still isn't ROSI, but it does have a nice shine to it.
Wendy Nather is Research Director of the Enterprise Security Practice at the independent analyst firm 451 Research. You can find her on Twitter as @451wendy.