How floppy are you talking? There are totally floppy dildos but I don't think they're used for much. Many dildos are made from silicone or rubber and have some flexibility to them but are basically rigid enough to ride. There is a fine line between having something that feels like a real human and something that is so hard it doesn't resemble the real thing at all (like glass - not made to simulate the feel of a human). Also, often times it's how the dildo is used - if it's specifically made to stimulate the g-spot it's probably more rigid. If it's for general penetration and to feel filled, rigidity is probably not as important. As long as it's hard enough to apply pressure to various areas it doesn't have to be too hard. Keep in mind, a real cock may be rock hard but there is still give to it - probably not really as hard as you think. I mean, if you pressed down on the head of your dick, your finger is going to sink down a tad - It's not like it's solid like marble where there's no give at all.
For me, no matter how soft, hard, warm, cold, etc. a dildo is, it's never going to feel the same as the real thing. In fact, it does little more than nothing for me.