Raw adjustment vs. using flash attachment

These are probably silly questions but I'd still like to hear forum opinions on it.

I'm thinking seriously about buying either the 430EX or 580EX flash. I shoot 99% in RAW and use RSP to process the images. So far I've only used the built-in flash on the camera when I need additional light.

The question(s): If I have the ability in RSP to increase the exposure level of an image up to 2 stops (give or take) for an underexposed shot, why do I need an external flash? Doesn't RSP do the same basic thing that an external flash unit would do for me (give me better lighting for certain shots than the built-in)? I.e., can RSP adjustments make up for the fact I don't use a flash attachment. If not, what else does the flash do for me?

I'm trying to think of a reason not to buy the flash and maybe put the money toward a 50mm f1.4 lens instead. I'm not sure which I could use the most (assuming I can only afford one right now).

Thanks for all input!

Can RSP "freeze" action in low light? Can RSP fill in backlit situations? Can RSP actually make and underexposed shot look as good as a properly exposed shot? I can't tell you whether to buy a flash or a fast prime, but they aren't mutually exclusive to better photography.

RAW converters may appear to be magical, but they aren't. They digitally amplify the light data recorded, and they amplify the digital noise, too. This gets worse as you increase the ISO and have more noise to begin with.

I'll add one more point to the excellent examples jfrancho mentioned. RSP can't fix the rather horrible effects of fluorescent lighting.

I consider a good flash unit to be an essential accessory for anyone serious about indoor photography.

any shots that are underexposed in the digital world is gonna give you a whole lot of digital noise when you bring it back to "mid-tone".

Yes you can use software like.. Neat Image and Noise Ninja...but the result will be nice and plastic with no details.

Shooting RAW means getting all the data you need for post-processing. sort of like negative film. The only thing with RAW is...it's boring and flat, until you digitally "remaster" it.

You need a minimum amount of flash to be able to get a good quality image when increasing the digital exposure with RAW. You don't want to not use a flash and pull the exposure way up - you'll get grainy/noisey image with banding. External flashes are good for distance since built-in flashes typically go 11 to 15 feet and the 550/580 goes 60 feet or so.

Get the 580. You will not be sorry. When I first started using it, I was blown away. Light is everything. But, just be aware that there is a learning curve to properly using the flash.

Ahh the next few months will be harsh, No money but such a desire for the 580EX :P

Thanks for all the good comments. Where is the best place to get the 580EX (i.e. best price)? Beings I take 90% of my photos indoors, I can see now where I could benefit greatly from a decent flash.