You only have to do it once, because once Lion is installed, it will put a recovery partition on the hard disk. So you won't need a disc after the initial install.
yeah, well, unless your hard drive crashes, or you simply want to put in a larger brand new hard drive, and rather than bulk copy everything over you want to do a clean install and start from scratch.
Besides: WHAT ABOUT THE NEW MACS RELEASED LATER THIS YEAR?!?! WHAT IF YOU WANT TO DO A CLEAN INSTALL ON ONE OF THEM, BUT SINCE THEY SHIP WITH LION ---- THEY WON'T BE ABLE TO EVEN BOOT IN SNOW LEOPARD!!!