For me the currency conversion, easy ‘totalling’, syntax highlighting, and mixing Markdown text inline (which gets ignored) is the key feature for these notepad/numi apps.
These still could be done in Vim but it’s more than just a unixy calculator or text-editor spreadsheet.
To each his own I guess. As far as I'm concerned, I'm mainly interested in the one-column spreadsheet, tweaking a few input rows and checking how it impacts some other rows. I still use instacalc from time to time, but now I often reach for vim instead.
These still could be done in Vim but it’s more than just a unixy calculator or text-editor spreadsheet.