I've battled with this myself for a couple years now. I am going back to school half time starting Monday.
Being a self taught software engineer, such as yourself, and making a pretty decent salary I've had a hard time really understanding what a degree could do for me. I get job offers left and right (and yes I ask for a fair bit more than the "average" salary for my area and fellow programmers tell me they want to work with me or convince me to come work with them.
Why do I need a degree? I'm creating a name for myself in my community, presenting at code camps and Alt.NET and being a leader, right?
There are a couple things that changed my perspective.
1. My Goals- I've been realizing I want to be a technical leader in my field. Most likely I want to pursue AI. I don't want to be wanted for my knowledge of a framework or programming language. I want to be wanted because I can do anything anywhere. I want to make the impossible possible. Well two things regarding this: First is that I've noticed that the most influential people I respect can back up their claims with formal reasoning and concrete evidence. I'd like to learn to do that better. Second is that innovators in computer science typically utilize a large foundation of knowledge to create new solutions. I'm thinking Google, Genetic Algorithms, etc.
2. Honest Reasons I Haven't Committed to a Degree Yet: Ego and Money/Time. Now that I make a good salary money is no longer the excuse. I've learned better time management as well so neither is time. The only thing really holding me back is that I was proud that I was self-educated up to this point. What I've realized is that I'll always be learning more than what I need. If I have a masters I personally expect that I'll be at more of a doctorate level. Getting a degree doesn't mean I'll plateau it means I'll sky rocket.
3. I took inventory of all the classes I'd need to transfer to a university. By this time next year I will have taken multi-variable calculus, possibly differential equations, calculus based physics, and more. This is just for my A.S.! I am excited to go even if I didn't get a degree at the end. REALLY excited. There's no way I will have taught myself all of those things on my own.
4. Whenever I want to brag about the place I work or my mentor (which is my boss) I mention that my boss has his M.S. in Computer Science. I finally realized that the degree does still mean something to me.
5. I don't like mooching. If I'm gonna self-educate I really need a mentor. But here's the thing, my current mentor has his M.S. and I would want future mentors to have at least that as well. So really... I'm admitting I want the college education but without paying for it and by having someone give away and devalue their time for my personal betterment. I just read Atlas Shrugged, and I don't want to be a moocher.
I'm only 27. So I figure even if it takes me 10 years to get through my Master's I'll still have plenty of time to enjoy that knowledge and leverage it to do some awesome things. For me it's not about money or fear of job loss, it's really just about I want to be as awesome as I possibly can be and just going for it.