It's cliched when it's not exactly true, and usually when it isn't exactly true, that seems to generally be the only reason people have because that's what they assume people want to hear. Deep down, the reason why a lot of the applicants (note "applicants", rather than "med/dent/etc. students") want to study med/dent:
1) Money (somewhat wrong take on reality)
2) Parents (worst reason)
3) It sounds "cool" (yeah, wtf)
And none of those reasons are the "right" reasons.
Also, "helping people" isn't really specific to medicine/dent either, could be used for nursing, working with troubled youth/homeless, teaching, and a variety of other courses.
Rather than helping people per se, I think it is much better to say that you care about people and their wellbeing and health. Otherwise, every single profession is helping people. This is because the reason why we do things is to, in some way or another, further society. Engineers help people by building safer structures, computer programmers help people by writing better and more efficient applications, a builder helps people by constructing houses.
So helping people isn't unique to medicine, but caring about people and their wellbeing and health is. You should also notice that medicine is one of the few careers where your life will be about interacting with others, if you're the type of person who likes a quiet office job, medicine isn't the job that you want, at all.
I agree with Rohit though, majority of people do medicine for those three reasons, as well as for power and chicks (thanks Russ).
Essentially, money isn't a bad thing, it's fairly common knowledge that doctors are well paid and very well off in society, they have stable employment on top of that as well. If money motivates you, I don't think that it will be too bad, but remember that you'll have to spend many years of being average before breaking through.
Parents, well, what can I say about parents. You have to think about whether you'll be happy being a doctor for the rest of your life because of your parents. Look, I know someone who wants to do medicine because of their parents and when I asked them this question, they said yes, they would be happy to be a doctor for the rest of their lives for their parents sake - so I guess that's fair enough then.
Well, it sounds "cool" - I can't say that it doesn't. But trust me, when you go out clubbing and you meet a nice girl and she asks you what course you do and you say, "Oh I do Medicine" - it's not the coolest sounding degree out there

(trust me, I've tried it, positive response 60% of the time - but the other 40% of the time, you get the "you must be a nerd" response).
People say doctors get the girls, but if that's your motivation, well, I don't know, but personally, I would think twice about dating girls that are doing med (for personal reasons as well as wanting a bit of variety as well) - so I guess doctors aren't for everyone.