As instructors we need to guard against the "perpetual student", that student who does not really care enough about their license to put forth the effort to get done with it but instead enjoys being able to fly solo every now and then for fun. The FARs do not prevent this and we have to watch for it as instructors. Making it worse is how hard it is to decide sometimes exactly who is serious and who is shining us on for the fun of flying without a license. I just had to kick someone out who had numerous 90 day renewals yet never made any progress and hardly scheduled any training flights. You have to ask them every flight what they worked on and how they are doing. There also needs to be a minimum flight frequency for progress to be ongoing. At least one flight a week is the minimum I use.