"[Summary (and YouTube links) of a debate on software patents in New Zealand]: "I argued that software patents stifle innovation. When ideas are copied, extended, built upon - we all benefit. Patents stop this and thus stifle innovation. He argued that patents not necessary to protect industries based on ideas e.g. publishing because copyright does a good job. He pointed to the negative impacts of software patents such as the Android litigation which is stifling competition."
Patent law may, indeed, sometimes serve to stifle competition. However, that is not necessarily the same thing as stifling innovation. In fact, it's possible that the existence of patents may in some cases encourage innovation, as innovators must think creatively to "invent around" patents and come up with something new.