The first time you get a spreadsheet that you can't open from a lawyer or accountant or send a spreadsheet that "doesn't work", it'll be worth it to have used Excel.
There's more than enough hassles involved with running a company - doing compatibility work for office software shouldn't be one of them - just use what is the industry standard.
I haven't tried Excel compatibility, but everyone is able to read .doc files, and yet compatibility with Word fails for almost every non-trivial feature (especially layout things). So, I'd be wary of that.
There's more than enough hassles involved with running a company - doing compatibility work for office software shouldn't be one of them - just use what is the industry standard.