I have a TAdvRichEditor hooked with a TAdvRichEditorMiniHTMLIO. I can paste plain text copied from Notepad for example, but when I try to copy/past text from a WPTools TWPRichText it doesn't work, yet I can paste that same text into Notepad. In fact, your OnPaste even does not even trigger, but does with plain text. The TWPRichText control is probably copying or identifying the copied text as RTF text, but it would be nice if your editor would still paste it. The popup menu even shows Paste as being available (not grayed out). Other editors allow the use of Shift+Ctrl+V to paste plain text. Maybe that could be an option.
I have no experience with TWPRichText. So, I'm not sure in what formats it can copy text to the clipboard.
TAdvRichEditor should be able to paste from RTF. To force TAdvRichEditor to accept text as only text, set
AdvRichEditor.ClipboardFormats := [cfText];
By default it seems to be set to:
But I did as you suggested and that solved the issue, thanks!