The orignial OP's instructor isnt wrong in what they say. And personally as long as the person is back at flight idle or idle power before lifting the noise through the horizon to flare I won't say anything. It does require more skill than keeping the power on and if the student is being forced to carry out 3 deg approaches it may make it nearly impossible. Which is another point instructors forcing students to fly C150's in the same style as a Jet with no doudt adding 5knts on to the approach speed due to instructor incompetence, 10 on for gusts and another 5 knts on for mother. Stick some power on in the flare and you have a whole heap of energy to get rid of that the performance never catered for.
MJ
While I agree with the general sentiment of your comments and I do come across people who fly their approaches way too fast, as you say having decided to add 5kts for this, that and the other over the years. It's not always wrong to add a sensible factor on occasions.
From the PA28 POH
In high wind conditions...it may be desirable to approach...at higher than normal speeds...
A common rule of thumb would perhaps be to add half the gust factor. The problem often is that once a PPL holder has done this, they "forget" to not do it for more normal conditions, but then add a further increment in the future and the effect becomes cumulative, then as you say they waste hundreds of metres of runway having to get rid of all that extra energy.