The option to Paste is actually based on the browser being used, and different browsers will allow pasting in different ways and from different sources. IE may allow pasting without any issues, while safari and chrome can only paste when you double-click the text box and bring up the cursor to paste only in that cell. Firefox allows you to "Paste Special" which is very similar to the "import from clipboard" option in the More > Import/Export menu, but can be done directly into Grid Edit.
Importing via clipboard will get you almost exactly the same functionality of pasting directly into Grid Edit without having to hassle with how your browser handles the Paste function.