One handy thing that I didn’t realise that you could do with Excel until the last few months was the ability to share an open workbook between users and collate any changes that are made (it seems that a form of version control is behind this). From what I have seen, Excel seems to manage changes to shared spreadsheets rather well. When you save yours, it adds updates from other users and warns if any edits collide with one another. To activate it in Excel 2003, all that needs doing is for you to go to the Share Workbook entry on the Tools and tick the appropriate checkbox in the resulting dialogue box. In 2007 and 2010, look for the Share Workbook icon in the Review tab on the ribbon to get the same dialogue box popping up.
That’s not to say that it doesn’t have its restrictions though and I have found that the merging of cells is made unavailable but that can be sorted by unsharing and resharing the workbook when no one else is using it. As to why cell merger is switched off by sharing, I have a few ideas. Maybe, they couldn’t make it work reliably (can happen with large software development projects like the creation of a new version of Excel) or decided that it would have consequences for other users that are too inconvenient. Either way, we cannot merge cells in shared workbooks and that’s the way that things are for now. Some may not worry about this though since they reckon that cell merging is undesirable anyway; well, don’t go doing it in any spreadsheet that is likely to be read in by another program or you could cause trouble.