as I see it the problem isn't using other people's ideas, it's giving people enough of an incentive that the develop those ideas to where they are useful.
the entire idea of the patent system is to make it so that those ideas are available for other people to use freely after the patent expires. If there was not such a system then there would be the risk of such ideas being lost, or locked up through trade secrets (with all the inefficiencies involved with maintaining such secrets, even where they are possible) for much longer time periods.
Our world today is what it is due to technologies that could have been locked up with trade secrets that are instead now freely available to use. Think of such things as how we make Steel or Aluminum and consider what would have happened if those were trade secrets of one particular company, with every employee being locked up under contract to not reveal the secrets.
Those are a couple examples where the process could be kept secret while selling the result. Prior to the patent system it would have been likely that such ideas would have died with the inventor, and had very little effect outside the immediate area (remember that if secrecy is your only protection against being flattened by your rivels you don't want to let many people in on the secret, which limits how large you can grow)
by giving out a temporary monopoly the patent system is designed to trade the short term advantage to the inventor for the long term advantage of society by making the inventor document the details of the invention. In the worst case (where the patent owner uses the patent to squash all use) society gets the invention a generation later, in the best case, not only does society get the invention free to everyone a generation later, the inventor is able to make good use of it in the meantime.