If they don't have WSH installed... the script won't work. There is no alternative for this particular problem atm, so it's better to make it work on some systems than none. Besides, if it doesn't work, no harm done, all the user has to do is press OK.
However, you do have a point about mass distributed scripts: there might be cases that the unexpected absense of WSH would lead to a 'major' bug in the script. So, the script should first check for the existence of WSH (there's $comerr and the Version property) before attempting to use it.