Having started working on the Hanon exercises only for a week now (my teacher assigned number 1 to me), I think that at least for some students, they are very benefical.
For students with small hands they help extend the reach between the fingers. Normally I can comfortably reach an octave in my right and a major ninth in my left; but now my right hand can easily handle a ninth and my left can now almost take a tenth. It has helped me in playing 4-part chords a lot easier, and 7th chords in root position. (which I have the latter in one of my exam pieces both in melodic and harmonic form)
Many pieces and studies require strong 4th and 5th fingers. I used to make higher notes on those fingers, especially in the RH, too soft, or even miss them altogether. My exam pieces and studies have been sounding at least 10x better since starting to learn the Hanon.
My goodness, wind players practice several exercises that don't involve actual music, but the exercises can be applied to written music. Some exercises are designed to help correct playing faults, or introduce a new playing concept. The Hanon, for example, can be used to correct problems with playing on the flat part of the fingers or playing with a flat hand.
So don't despise Hanon. Or many other exercises either.
Meri