I agree with you Deltaone that there are some cr*p radios out there, and as has been mentioned you do get accustomed to being able to decipher it. Some pilots with bad radios are better at getting their message across than some pilots with 5x5 transmission quality but who tell you everything except what you really need to know, but I digress...
I think the 1-5 readability scale is good as it saves people describing your transmission with a lot of words whereas the scale is at least concise. It is subjective though. After years of listening to HF in a remote corner of the globe, where there the best readability you gave anyone was 4, VHF even at its worst would probably get no less than a 3 from me, but maybe 1 from someone else.
As far as what can be done about it, well you give pilots a radio check and unless it is totally unreadable you are hardly going to send them back to the apron and spoil their day (talking about club flying here). Maybe we should phone the club/school concerned but I for one usually forget!!