There is no competition here. Use what you prefer, but the BTZS file does not do the same. It does not work with 31-step tablets and requires much more user interaction (more inputs), and I did not find it that easy to use.
With the WBM spreadsheet, you enter the data on the first page, enter the effective film speed on the last page, and you're done. You don't have to read anything off the graphs and enter it numerically (except for once). This relies on some matrix evaluation in Excel and is probably the reason that Open Office doesn't work with it, because it does support it. The final results are also not the same but very similar, except for film speed.
I was asked to do this, and I did. It is not a competition to BTZS. It's a different approach, but you are right, it needs more documentation.