Why do you discourage people from playing them? You are always saying that one should only play what one loves. 
Because I am in principle against arrangements. One should play what one loves as long as it is the original piece. As far as I know neither Debussy nor Tchaikovsky ever wrote a piano version of the pieces, so chances are that s/he will end up with some facilitated non-idiomatic and awful version of pieces that wound wonderful in their intended set-up, but sound terrible in the piano. Just get any beginner’s collection of pieces and sight-read through the versions for Beethoven’s “Ode to Joy”, for instance.
I do not know the level of the person asking the question. But most adult beginners who spend time and effort learning things like easy arrangements of “Ode to Joy” live to regret it once they realise how impoverishing musically such arrangements are. Then they wish they had used their time by learning instead an equally easy piece but far more satisfying musically (since it was originally written for piano and for that level).
Given the huge amount of pieces originally written for piano, I see no need to play “Ode to Joy” or “Waltz of the flowers” or “Prelude a l’apres midi d’un faune” on it. I am sure s/he can find amongst real piano pieces some that s/he loves just as much or even more.
Best wishes,
Bernhard.