As far as "...I cannot see any way to make Forms insist that text is entered in the other text-box.". There is a way to make Forms insist that text is entered in the other text-box.
Mark the question as "Required".
As far as "...usually don't show all the "other" responses as they are...".
Google forms always handle Multiple choice/Checkboxes/Drop-down questions the same way.
To categorize, elaborate and clarify on the form reactions.
Not required questions:
- The form does not "care" whether some input in the "Other" field exists or not.
- The form will record the "Other" field, only if there is input in it.
- The form will not record an answer when the "Other" field is selected but left blank.
- The form will not record an answer not even if some blank spaces are entered, when the "Other" field is selected.
- The form will force an answer to the question.
- The form will accept/record the "Other" field, only if there is input in it.
- The form will not accept (let alone record) the answer when the "Other" field is selected but left blank.
- The form will not accept (let alone record) the answer not even if some blank spaces are entered, when the "Other" field is selected.
An easy way to understand how a Google Form works, is to think of the "Other" field existing in both the Multiple choice and Checkboxes as the option "choose" in Drop-down lists.
The added extra that comes with Multiple choice and Checkboxes is that a submitter is allowed/forced to provide one's own input as an answer. So when one selects "Other" but provides no input -or just blank spaces- there is nothing for the form to record (as if it was never selected).
Blank spaces entered
You can also have a visual understanding by these screenshots of the "SUMMARY" tab:
When a question is not marked as "Required", you notice that although there are three responses submitted, there are only two answers recorded for the specific question and just one of those answers recorded for the "Other" field. That is because despite the fact that two people may have selected the "other" option, just one of them also provided some input for the field. Input you can see recorded under the "INDIVIDUAL" tab as well as the connected spreadsheet.
You can further verify the results by checking the "INDIVIDUAL" tab for each responder.
Keep in mind that submissions (as "entities") are stored within the form. Spreadsheets only display the submissions after a respond is submitted and only if they are already connected to the individual form. (That is exactly the reason why when even if you delete a submission from the spreadsheet, you still have to delete it from within the form.)
Therefore, if an answer is not recorded by the form (no selection/selection carrying no input/blank spaces), even though you have the submission, there is no data to display for the specific answer in the spreadsheet, resulting the corresponding cell to be blank.