Rotating two chains on one cassette
There is actually a real gain from this strategy. By rotating 2 (or 3) chains you extend the life of rear cogs (cassette) and it will last all your chains. In case you use one chain, you may need to replace cassette together with chain.

As of front cogs, they last longer, but you will get a more even wear.

Speaking of drawbacks - regularly taking off your chain you can make your PowerLink (or other brand) a bit more loose and have a higher risk of it failing under a load.