The big reason I'm "considering" (once again, just a possibility) getting a higher speed disc is that the courses I play are 9-hole par-3's that have hole lengths from tight-curving 245ft holes to narrow 370ft holes, and even if I get my mids to go straight, a 170ft shot on the fairway just isn't leaving me in a good place to par, more-or-less birdie. I typically end games 10+ through 9, but now that my shots are going straighter and further, I am sure that is bound to change. Basically, I'd want to have a disc where I can get some distance on those long, 340-370ft holes and get a good, further lie on the high-200ft ones. I know with time that my mid distance will improve, but I'd also like the possibility of having a far but slightly off-target starting drive so I can just "get it closer" on my second shot, rather than trying to approach it from a bit out. Does that make sense?
Once again, I am not neglecting my mids at all. My mids are my favorite and deadliest discs in my bag. I'm just saying that in-time, adding something like what I mentioned to my arsenal could provide a good and suitable use.
I think there's no problem buying some higher speed discs. You will grow into them, and some are not bad as a beginner. Actually, even in the canonical How to Build a Bag, garublador recommends a speed 9 Firebird as part of a beginner's 4-disc bag.
I'm a believer in the "play disc golf to have fun" philosophy. Personally, I don't play disc golf to attain that "perfect" round that will happen when I can throw 450' controlled and make every putt inside 60'. I am happy with reaching for "perfect" relative to my skill level at the time of my round, and then I simultaneously try to improve my game (form, putting, etc.) when I can.
The key, I think, especially in the beginning, is just making sure you don't reinforce bad habits while you are having fun. Keep working on your form and throwing those putters and mids, but don't let us make you feel guilty buying a couple drivers. I would, however, recommend you keep it to speed 9 and below at first.