People don't argue that patents are good for consumers because they aren't. People argue about if they're good for inventors and technical progress; if inventors will only innovate and share the results with the monetary benefit provided by a patent.
And the point of technical progress is to benefit the public (i.e. consumers) in the long run. So it all comes down to that.
The issue is in the implementation and outcomes. As you say, do inventors only innovate+share when provided with the monetary benefit of a patent? It would probably depend on the field, but I suspect innovation would still occur even with shorter patent protections for software design.
People still innovate even when they know they're gonna get copied. Maybe not as many people, but it doesn't make as big a difference as the pro-patent lobby would have you believe.