Yes there's something not quite satisfactory about handing over your IP to Turnitin with little choice in the matter. I think in the end though I concluded that

Yes there's something not quite satisfactory about handing over your IP to Turnitin with little choice in the matter. I think in the end though I concluded that for honest students like us the pros outweigh the cons. I'm so sick of plagiarists in higher education. Ironically Turnitin ended up costing me dearly - it shows your marker a percentage of how much of your essay consists of quotations. I was penalised for breaching the maximum threshold of original content to citation, my grade capped at 59/100. But I doubt the marker would have realised otherwise, because it was a well written report (about database copyright) that flowed naturally; I simply included longish quotations from my sources to make things clearer. Being told to 'learn how to paraphrase' rankles to this day. Argh. Maybe I can have the last laugh by making it my glorious blogging debut...