I fear that JavaScript's use of floating point as its single numeric type is likely to create surprising results for the unwary. I have come to the suspicion that the longevity of Excel's dominance is in part due to a sophisticated system for inferring numeric types.
There is some fancy logic to format numbers, however (including some mess for date codes). http://oss.sheetjs.com/ssf/ is a pretty decent implementation in JavaScript.
http://msdn.microsoft.com/en-us/library/office/bb687869%28v=...
Many people would be fine to use other spreadsheet software but why bother going through the bother of researching and downloading and learning that software when your company probably has Excel licences and you've got some entry level training that used Excel?
That dominance has other effects. There is a huge amoun of support (training courses; books; Usenet groups and forums; Magazine articles; other people in the office; etc) for Excel that don't translate immediately to Excel. There are very many third party tools to help you do stuff in Excel if you really need it to. And Excel is probably some people's first introduction to programming. (Which is perhaps a bit scary.)
I speak as someone who loves gnumeric and LibreOffice btw.
Huh? Doesn't Excel just treat everything computationally as doubles and sometimes display them as integers? As far as "sophisticated systems" for dealing with numeric types go, that's fairly weak sauce (and quite easily doable in JavaScript, which already does the first part).