It speed up addons when its building metadata. The same that is happening when you build a library. Building of my library went from 2 hours to 20mins. The same will happen in Ic*f*lms, 1Ch**nel, movie2k, movie25, TV-release, etc. so it's when building caches fetching new metadata so you can imagine that you really will see a big difference here (it's about as fast as a PC fetches it) so I would consider this the main benefit.. It will also make some external skins run a little faster but these are not the problem really.
As a bonus the fanart will also load instantly when you browse movies ie. I guess you can say the last real bottleneck is lifted. It's all been well explained, and RPi is a tinkering device I would never go out and buy expensive new hardware explicit for XBMC on RPi. As said I'm using a old 1.8" mini harddrive I had laying around. The USB sticks I had was to slow and if I was gonna buy use something else I probably would have waited untill a fast usb3 stick fell in my hands for a reasonable price so I allways could use it for other projects on RPi or another device or used one of the existing SSD device's I own.
Also I never installed it for a friend this way who did not have the parts laying around already.
As said all that we are talking about is a bit geeky but most of us love it. You could settled with a sdcard only and no tweaks but that's not what makes it fun to me. The budget is still small regarding the RPi for me and I have no dents on my budget at all.
I like my rpi's for their versatility as said one of them is gonna run my server and it sure is gonna save me some power, noise, and whole setup in general. I plan on running it headless from a 3.5" external HD, with a LCD display showing load and bandwidth throughput. Again as a hardware geek I already have some displays, electronic components and various non RPi but fit able cases laying around.
Regarding how open and hacker friendly this device is its still the best 30£ I've spent in a long time there really is no obstructions to make it do what you want and that's want I like..