Lobbying is very successful in policy making. Lobbying is a legal form of bribery but it is very evident and effective within the government and it defininately infuences policy making because, in return for large sums of money, policy makers are paid off for passing and introducing policies that lobbyists are asking for.
As stated before, it is legal bribery and another way that the rich can use their money as power to control the government. The cause is sometimes a good one but the way they go about it, through lobbying, is not very ethical in the long run...it just happens to be legal.
Private interests are about to legally influence government decisions which doesn't seem like the right way to go about things. Why not just vote on legislation and policies instead?