Not wanting to get off track because "degree or no degree" has been debated ad nauseum elsewhere on these forums, but in my case going to university definitely helped. These comments only really apply to people coming out of school rather than people looking to become mature students... Not only does it give you experience of living away from home before you venture to the other side of the world for the best part of a year, but it also forces you out of your comfort zone of home/school life and makes you more socially aware. Getting a good degree proves you are capable of studying a subject at a higher level - remember, getting your fATPL involves a lot of study and the FTO will want to see evidence of your capability to get your head down and work. It doesn't have to be aviation, maths or science related - I actually did a maths degree but not one bit of it has come in any way useful for my flying career. It's more to do with the experience it brings. PAJ's bullet points sum it up perfectly.
If you do end up without a flying job straight after training then I would suggest that having a degree would open more doors to you, not fewer. I don't know of too many people who struggled to find work in these circumstances, and the vast majority have degrees. What's more likely to be an issue is the fact you have "aviation" on your CV - potential employers might be scared off by the knowledge you're bound to jump ship back to a flying job as soon as one comes along, unless you went into a job on an agreed temporary basis as I know quite a few have done. I wouldn't, however, advocate DJ's suggestion of CV-doctoring.
Although some study of maths or a physical science might help you a bit, degree level stuff tends to be far more advanced than is required for ATPL study. I found A-level maths and physics helped me understand some of the concepts, particularly in Navigation and Meterology, but it's not utterly necessary. As I said above, as long as you have the work ethic and the ability to study properly, then the degree you go for doesn't really matter.