Could you exactly say how big your hard disk has to be at least when the vendor claims it’s 500 Gigabytes big? Do you know how many bytes a file has when the filemanager says 2 Megabytes? Do you know what your DSL-provider actually means with 2 Megabit per second?
Experienced users surely can since they know the meaning of “Kilobyte” depends on the context. Sometimes it’s 1000 bytes, sometimes it’s 1024.
Non technical users will tend to assume that a 1 Kilobyte is 1000 bytes (which is correct, since k is the SI prefix for kilo) while users with technical background will tend to assume (depending on the context of course) it’s 1024.
There is a solution for this ambiguity, but unfortunately it doesn’t come for free: First, it sounds really stupid (kibi-what?) and second it will add an extra character (kB -> kiB) and we all know how stuffed our 80 column terminals already are nowadays.
If you have better arguments against the binary prefix notation please, join the discussion. Personally I’d find it very cool if we could use SI prefixes where we mean n * 10\^3 and binary prefixes where we mean n * 2\^10 in Debian consistently.