If you don't take care of your IT systems - whether utilizing internal or external resources - don't be shocked when your IT systems experience issues. Technology has come a long way in just the last 20 years, but the systems that companies use every day are still rife with issues and require routine maintenance. Now, even maintenance won't prevent unexpected issues, but it might discover impending doom before it strikes (think of it as Spidey sense).
Don't mean to be redundant (you've read this next line in previous postings), but regardless of your line of work, your business runs on technology. Phones, e-mails, BlackBerrys and iPhones, financials, etc.; it's all dependent on your technology working. So, don't think you can wait until it breaks to fix it; technology has a notorious history that once it breaks it's broken.
I'm not advocating going crazy with your IT budget, but a certain amount of spend is required to maintain your systems' integrity. How much depends on the complexity of your environment, but use history as an indicator. What you spent in the past can to prelude to what you'll spend in the future. If you have a good team - again, internally or externally - then you can trust in your estimation of the future costs.