I've dealt with enough developers that I'd say about 1/2 to 2/3 were competent, and some of those, while competent developers, didn't have anywhere near enough knowledge to be allowed to administer their own machine.
Of course, I'd probably say the same about 1/2 of the sysadmins I've worked with as well, and very few sysadmins have the knowledge to be competent developers. The higher percentage of incompetent sysadmins stems from not understanding security, or not understanding how to balance security with usability.
Note my use of the word knowledge -- most developers could be competent sysadmins, but don't have the knowledge base. If you're developing database-driven web applications using .NET, how likely are you to need to know details of filesystem ACLs, or how a rootkit may insert itself in to a OS kernel? How many sysadmins are going to know how to write (or even use) a database stored procedure, and more importantly, /when it's appropriate to do so?/
Unfortunately, most people don't have the wisdom to understand where their own knowledge ends. I understand quite a bit about a lot of topics, but I'm willing to let an expert (doctor, dentist, mechanic, athletic trainer, etc.) do their jobs. Why don't people have the same attitude about sysadmins? (Hint: I think it's because when you fuck up your car, you pay to fix it. If you fuck up your PC there's no direct financial penalty for you personally.)