The DPDT switch will serve the same purpose as the reversing module, it will switch the polarityof a reverse block. I assume you want to use the switch rather than a reverse module to save money. What you are giving up is automatic operation. You need to manually ensure that the polarities match entering and leaving the reverse block. Just flip the toggle while in the block, making sure it was set correctly when entering. As already stated, the polarity means nothing to the loco direction. The main drawback is that when you forget to match the polarity, the breaker in the booster will trip, shutting down the layout. I assume that if you are saving money by not buying a reverse module that you are also not going to buy power shields to section your layout. While the short will not cause damage to decoders normally, I think its a good idea to minimize how often they occur. I have had to reprogram an address in a loco on occasion. Rarely tho.
Gary