Thanks for the suggestion! I know the feature you’re talking about on Google Drive/Docs/Sheets, and it does come in useful sometimes. This probably won’t get added to our comments in the very near future (it’d require some cross-platform work), but I’ll pass on the suggestion and add it to our list of possible improvements.
Thanks for the detailed thoughts! Building an importer is definitely possible, but it just hasn’t been a high enough priority for us yet – only a handful of people have asked for it, and we’re a tiny team (2 developers), so other updates end up taking precedence. We might get to it someday, but I wouldn’t expect it any time soon, unfortunately.
If you’re feeling brave though (and you’re willing to ignore XKCD’s advice), we do have an API you could use for importing! Docs are here: http://dev.splitwise.com. Maybe someone will put a nice CSV import program up on GitHub for the power users :)