If your website is mission critical but server administration is not core to your business then don’t bring System Administration in-house. It makes economic sense to outsource the function even if you run multiple servers. You contain your costs with one less full-time salary to pay and one less person to manage.
You want 24/7 quick resolution to problems. The minute your web server is not operating properly you want a skilled person diagnosing and resolving the issue. You want it to happen immediately – preferably before you or your customers even notice.