If you think that it makes sense to give the first person to come up with an idea exclusive rights over it for 20 years, compare the amount of innovation supplied to the world in having the idea of putting something interactive on the Web to the amount of innovation that has been built on interactivity on the Web (and yes, the patent is very close to that broad). The former is hardly trivial - it was a novel idea and would have been a very cool demo at the time - but the latter is half of the technology industry. Consider the tens of millions of dollars Eolas was able to extract from random companies while doing nothing, and how much more they might have gained if they had happened to have been a few months earlier. Consider the stupid hack required for years to work around the patent in IE - it could have been worse, since this patent was broad enough that nobody was going to stop innovating because of it yet not broad enough to lack a simple workaround, but it was still stupid.
I have a hard time seeing who would be harmed if software patents were categorically invalid.