More than business
In software, patents don't just affect businesses.
 Practitioner profile
(Can you help? this section needs review. I just copied it from Abandoning software patents? (with permission from the author, me.))
To see how different software is from most patentable fields, you just have to look at the practitioners. There is a small number of large companies with well known products, and there is a mass of small companies. The low cost of entry to software development means the number of small companies is particularly large, but we'll leave that aside to look at a bigger difference. In most patentable fields, this pyramid of big and small companies describes how products are made. If this were true for software, then the decision of patentability would be an economic decision, and some costs might have to be reduced, but there'd be no fundamental incompatibilty. But in software, this is only half the story.
In software, unlike in other patentable fields, there are two additional categories of developers. The first is the software developers that sit in the IT departments of every medium sized company. They're the folk that keep the emails flowing, who make internal software, extend software bought by the company, and who run the website. The second group is individuals, informal groups and communities who program for their own benefit or for social reasons such as providing alternatives to software seen as overly restrictive.
The existence of these two categories changes everything because it's obviously unreasonable to require them to work within the patent system, and it's unjustly restrictive. Not only are patent incentives obviously not necessary to motivate IT departments to fix problems, the timeline and budgets are orders of magnitude out of sync with the speed and costs of writing software. When a company manager reports a website problem, they don't expect the IT department to reply about first seeking legal advice for a patent search, and they don't expect to later have a bill from a patent holder because of the way in which the IT department happened to fix the problem.
For user communities programming to suit their own needs, the costs and timeline are also unreasonable, but there is also the bigger problem that the patent holder gains veto power over the distribution of the software. If the software is written for the purpose of having a freely redistributable program, then this third-party veto spoils the developer's efforts. There will be no direct profits from which to offer royalty payments, so the result is a lose-lose situation where the developer's goal is blocked, and there's isn't even anything in it for the patent holder (although the patent will still be enforced to sink the piece of software so that computer users are pushed toward a program which will pay royalties to the patent holder).
 Related pages on en.swpat.org
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