MP3 is a "lossy" format. Like jpg graphics, the lower the rez, the more is lost. For example, let's say you start with a decent quality visual image, say 300 dpi or more. With jpg pictures, if you are viewing on a computer monitor, you won't notice much degradation from the original at mid-rez, because the resolution of a monitor is only around 100 dpi (more or less). But if you are wanting to print the image in a publication or project it on a screen, you would notice it.
With audio, your audio playback is the deciding factor. It all pretty much depends on the sound system you plan to listen through. If you listen through little headphones or your average ordinary car speakers or a boom box or ordinary computer speakers, mid-rez is fine. If you listen through a quality stereo system, mid-rez will have hard edges, less transparency, and suffer from loss of dynamic range. If you have a for-real hi end audio system, mp3 sux, so don't bother. It will hurt to listen to it.
Just as with jpg images, once the detail is lost, it cannot be recovered, and recompressing into another mp3, even one that is set for higher rez, will actually further degrade the audio.
Which is why I never download low rez, and avoid mp3 whenever possible as my original source. If I have a high-rez original (like the CD original), I can always rip and burn low rez mp3s for other purposes. But if all I have is a crappy low rez mp3, it is of very limited usefulness. I still do all my serious listening through a quality stereo system.