I am mostly of the same opinion as you are, but really what they are offering is an easy way to expose your data source as a service, and to provide a simple already established user interface over that data source.
So it does have its place but probably has a limited sweet spot. Is your end user already familiar with spreadsheets, possibly might want to view/edit the data in bulk, not going to pay you to build ur own editor over the data, not really that concerned about performance on a large scale? Then this might be a good solution for them.... Its basically MS Access in the cloud...
I agree with you in that it's not a bad idea for your use case. I would likely still make an admin portal to insert data. If we are dealing with static pages that have a dynamic spreadsheet, I would look up a Wordpress or Drupal solution first.
After reading your comment, I'm thinking that it may be easier in some situations like a teacher posting curriculum. It's also much easier than setting up a Wordpress deployment.
So it does have its place but probably has a limited sweet spot. Is your end user already familiar with spreadsheets, possibly might want to view/edit the data in bulk, not going to pay you to build ur own editor over the data, not really that concerned about performance on a large scale? Then this might be a good solution for them.... Its basically MS Access in the cloud...