This is a good point, although one important consideration is level of risk. With margin lending, isn't there significant counterparty/exchange risk? If the site you're lending on goes Gox, then you've lost everything. Dash has price fluctuations, but I would think is more secure due to the lack of these risks.
When I said there weren't many places you could invest your holdings and get a return on them I was referring more to asset classes. But it's true you can always lend them out and get a return.
Indeed, if the exchange goes on the blink, then I've lost it all. But then again in this way it's similar to hosted pooled MN services--counterparty risk. If they decide to take my dash and run, I can do shit all about it (unless there is some sort of governance model that is being used with multi-sig protection). Me trusting the exchange to not go under is similar to trusting the hosted Pooled MN service.
@Tok: Isn't holding and lending BTC an asset class in itself? With a dash MN you are essentially doing the same thing--loaning out your Dash to provide MN for the network, as well as enable InstantX.