There is a good sermon that was given on this subject actually:
But, basically, Jesus didn't want the in-fighting, which may be similar to having more denominations, it depends on how you define denominations, as you will see that there may be disagreements within the same denomination so we have fighting within, even within the same physical church.
For example, there are people that claim to be following Jesus but then opening profess hatred of others. This we need to stand against, as that violates what Jesus taught, and invalidates his entire message.
But, we shouldn't be fighting against each other, and using valuable resources/time, when there are so many things in this world that could be helped if we focused on our mission of sharing Christ with the world.
Now, if someone is claiming to be doing work in Jesus' name, and you feel that there is some teaching that is suspect, then go by yourself, first, and try to share your concerns with them, but in all things it should be done with love.
This follows the teachings in Matthew 18:16-20.
By going by yourself, your ego is less involved, if you haven't told anyone what you are going to do, but just do it in a way so that only God is the other witness.