Well, let's take a look, what are the advantages of having a choice of engine?
The A320neo is set to offer engines from two different manufacturers: the CFM International Leap 1A, and the Pratt & Whitney PW1100G. These families of engines are designed to be more fuel-efficient, and what ever more "efficients" there are.
How does adding more options for engines for potential customers act as an advantage for the Aircraft Manufacturer?
Let's take a look at this: My friend lives in New York, my cousin lives in Amsterdam, I live in Toronto. What happens if I want to visit any of them, what happens if I want to see all of them? That would be some very expensive flights...
Airlines don't want a wide variety of things in their inventory either. Take a look at maintenance. Some people can work on one engine, while other people can't. Then, you might have unfamiliar technologies. There would be a lot of change, I'd need different people for each engine. That would be very costly too.
For airlines that already have a "predominant" engine in their inventory, they'd want to stick to it. They're familiar with the technology, the basic systems, and they have engineers who can work on this too. This is why many airlines stick to a sole engine manufacturer, or more importantly, a single airplane manufacturer.
Having different choices means you can serve a wider market. If I see that I can get this plane with my favourite engine, it's a double win! :)
Why wouldn't the manufacturer optimize its plane for one engine specifically, to give best performance and roll out the plane like that?
Many manufacturers already do. If you were to look at the Bombardier CSeries, the exclusive engine is the PW1100G. Engines aren't the biggest part. If an engine can do it's job, deliver fuel efficiency and so on... Then why shouldn't we have it as an option? If it's feasible to have more than one engine on a plane, then they'll go for it.