You're probably talking about the XFG. Most of the sets do, indeed, use hybrid tyres on the rear wheels, because they're easier to get up to temperature than normal road tyres. A tyre that's up to its ideal temperature offers better grip than a cold tyre, so there's your reason. I don't think I've seen setups that use hybrids on the front (and driven) wheels, or on all four.
It's perfectly acceptible for online racing. I don't know about real life racing, though.