Quoted By:
>New region
>decide to catch a new cool looking pkm
>use your one use only TM on it
>realize a couple of gyms later that said pkm is garbage with a shit moveset
>replace said mon with a more competent one that ends up being a pretty decent pick
>well too bad you can't teach it that useful TM anymore
Also, unlimited TMs allows you to use more pokemon and make them useful instead of the same six for the rest of the game because those are the ones you used your only limited TMs.