It would be very useful to delete the category for multiple book entries at once, e.g. after importing a lot of entries, where the category has not been assigned correct, and you want to uncategorize them for now. It is possible to assign a new / another category to a selected list of entries, but I've found no way to delete the category, besides when editing a single entry: the category field is empty and there appears to be no way to say that the category should get unset. I have also looked into deleting the category as a workaround, but then KMyMoneys requires you to assign another one.
You cannot have transactions without an assigned category, that's why it prompts you for a new one when you try to delete it
What you would like to achieve by deleting the category? As Alvaro said eventually you'll need to assign a category to every transaction so if you just need some temporary marker to know which transactions need processing then why not use a category for that. Let's say create a category named 'UNPROCESSED' and assign all these transactions to this category. Don't you agree that this approach equivalent with having a feature to delete categories from multiple transactions?
Yes, using a category like "UNPROCESSED" is basically what I wanted to achieve. And since the search in the Ledger View also looks into the category field, this works for me. Thanks.