Great option for players, terrible option for Riot's business plan. If you could trade champions with a friend, it would highly de-value IP because you wouldn't need to purchase any champion that your friend has. With de-valued IP, you'd see other IP prices inflate to compensate for players not spending as much IP. Rune costs would rise, as would things like rune pages and boosts.
You get 10 free champions every week to play. If you like one, help Riot by purchasing it in some way. Don't look for these handouts because you simply cannot earn enough IP to play all the champions you want. That's why you can buy RP -- to instantly obtain any champion you want.