Well it doesn't help more than the autocomplete function for the nicknames does now. But I use this function quite often and in my opinion it's a nice thing if it would also work with the words written recently in the chat window. I would rather write them out than copy them (although it's quite easy to copy in mIRC). But I would rather press tab than write them out if this function existed.
But I admit, the english language has mostly small words. My mother tongue german has a lot of longer words like "Empfangsdame" oder "Lokomotivführer". So it would be much easier to type "emp[tab]" or "lok[tab]", after the words have been mentioned by somebody else.
Your suggestion to press ctrl+f didn't correspond to my idea. Technically ctrl+f does the same, but I just said this to point out that the autocomplete function is very fast. The autocomplete function has to scan backwards to get most recent words first.
Script and IRC related messages are also processed, and I don't think this is a problem. You can cycle through the words when pressing tab again, so if you get the wrong one, press it again and you get the next one.
Like I said, it's a small feature. Just an extension of the autocomplete for nicknames... I must be crazy to discuss about it like this