Software stands out among fields of patentable invention because many---probably most---software practitioners oppose software patents.
Todd Vernon, CEO of Lijit Networks, had this to say in celebration of finally being granted a patent on software that broadcasts conference presentations:
Having spent what seems like millions of hours constructing [software patents], baby sitting them, defending them; it is really all wasted time and effort …. There is no way for a software engineer or system architect to have any idea what exists out there to either copy or avoid (whatever the motivation). The process is protracted, and in this case eight years have passed – what about the poor guy that thought up the same concept 1 year after I did and has made 7 years of revenue with it? Should he now have to license that idea? Should he have to back pay on 7 years of revenue?
I can say having seen the process pretty close that I have no doubt that most of the key patents out there around "computer screen sharing" (used heavily in web conferencing) contain a good deal of the same concepts and they all have been awarded over the last 20 years. They simply claim the same things. Or, at least they don't meet the bar of being unique and unobvious.
So, why author patents? Because everyone else is…. 
Perhaps software patents aren't for software authors at all, but for the venture capitalists who fund small startups. Brad Feld is such a person: his venture capital group helped to found FeedBurner and NewsGator, for example. But he too sees more harm than benefit to software patents:
I’ve skimmed hundreds of software patents in the last decade (and have read a number of them in detail.) I’ve been involved in four patent lawsuits and a number of “threats” by other parties. I’ve had many patents granted to companies I’ve been an investor in. I’ve been involved in patent discussions in every M&A transaction I’ve ever been involved in. I’ve spent more time than I care to on conference calls with lawyers talking about patent issues. I’ve always wanted to take a shower after I finished thinking about, discussing, or deciding how to deal with something with regard to a software patent.
After wrestling with software patents for the past 15 years, I’ve concluded that there simply is no middle ground. If we continue on the path we are on, patents will continue to increase in their overall expense to the system, everyone will feel compelled to continue to apply for as many (and as broad) patents as possible, if only for defensive reasons (one of Fred’s VC Cliche’s of the Week was “Patents are like nuclear bombs, you just got to have some.”) Let’s take a page from geopolitical warfare and focus on global disarmament, rather than mutually assured destruction. 
Art Reisman, software patent holder and CEO/CTO of AP Connections, arrived at the same conclusion to which many others arrived: that patents make sense for innovation in the physical realm, but not in the world of text and equations:
If you flash back to when Einstein was working as a patent clerk, everything that came across his desk was a physical invention with levers, gears, whistles, blades, and wheels. There was no way to protect this property without a patent because anybody could look at a production version and copy it in a machine shop. Somewhere along the line things have gone seriously awry and unchecked.
Here is my recommendation for some simple rules that could be used as a patent litmus test.
Patents should be allowed for:
* 1) devices with mechanical components
* 2) physical compounds that can be weighed on a scale.
Patents should never be awarded to:
* 1) Ideas
* 2) processes, recipes, software programs
I believe that these already have other appropriate means of protection (trade secrets, copyrights). 
And there is much more to be had, such as a lead developer at Apple who holds patents, is generally pro-patent, but acknowledges that patents no longer serve their core purpose: "I don't think that patents foster innovation … I don't think Apple or anybody's products would be dramatically different without the patent system."
Or this attorney who acknowledges that software patents are more about making money for interested parties than fostering innovation. He describes IBM's use of its patent portfolio as a "shakedown."
Or remember the infamous "One-click" patent from Amazon? The first programmer to implement it feels that the entire patenting process was a waste of time.
Want more? This presentation by Daniel Ravicher of the Public Patent Foundation includes still more comments from practitioners from Adobe, Cisco, and Oracle about how software patents hurt the industry.