Dan has some history here. As the creator of VisiCalc, he's previously written about why he didn't patent that software. His latest rejoinder is in reaction to:
Russ Krojec's blog entry "What if VisiCalc was Patented?" Russ argues that since VisiCalc wasn't patented competitors found it "...safer to copy the currently winning formula and avoid having to innovate. In this case, the lack of patents brought innovation to a standstill and we are all running spreadsheet programs that still operate like 25 year old software."
Dan correctly points out that Russ' position just doesn't square with historical facts--and demonstrates thereby the value of having an understanding of history. Back in the DOS days, I clearly remember any number of programs that were perhaps inspired by the spreadsheet metaphor but deliberately took different paths. There's some discussion of different approaches and programs here. A number of the alternatives were more explicitly multi-dimensional or iterative or capable of solving more flexibly-designed equations than conventional spreadsheets, but none were particular successes. Products that were very direct VisiCalc successors (basically Lotus 1-2-3 and then Excel) ruled instead, but it wasn't because there weren't alternatives. How come?
One reason that I'm pretty confident in giving is that, as the PC became more widely used outside of hobbyists and specialists, this forced a certain regularization of applications, for lack of a better term. Suddenly, the computer unsavvy needed to use these apps. A whole ecosystem of specialized training classes, books, and support systems sprung up. This tended to marginalize mainstream software that broke with established models. It was hard enough to teach people new command codes (which were highly irregular in the days before Windows), much less radically different mental models for how softrware worked.
Another reason is a bit more philosophical and I'm correspondingly less sure about it. But, perhaps the spreadsheet was just a metaphor and model that really worked and connnected with people--and the alternatives were just more copmplex variations on a theme that generally detracted rather than improved. In fact, most of the "innovation" around spreadsheet replacements has since been replicated in mainstream spreadsheets (e.g. for multi-dimensional, think pivot tables). And a tiny percentage of spreadsheeters use any of these capabilities-at least on a day-in, day-out basis. Furthermore, like word processors, the spreadsheet had a familiar physical analog--the accountant's ruled sheet.
I'm not convinced that software patents are bad in toto, however flawed the current system may be. But to say that more patents would have spurred greater invention in desktop productivity software just doesn't have a historical basis. And, indeed, at least with the reality of today's overly broad patents, it seems likely that just the opposite would have been the case with every remotely-related product litigated.