It adds probably 1-2 quarts to the cooling system and adds a fan so running a heater core will give you some edge in a marginal cooling system on a hot day. As far as rotting the floor out, that won't happen, because antifreeze has rust inhibitors in it. Eventually the coolant evaporates, even if you don't dry the carpet, done it many times.
If you're in a climate that needs a heater, then the coolant should have antifreeze too, not straight water. It's also smarter to run a rust inhibitor to protect the inside of the cooling system against corrosion.
Yes, I have bypassed heater cores as a temporary measure to get home, and get a new core installed. Heater cares are easy on some cars, and incredibly hard on others, A/C usually complicates installing them. I've also dumped pepper into the cooling system for a temporary stop leak, it usually holds about a week. Use Bars Leaks pellets if you want it to last longer. It doesn't clog heater cores, because I have it in several of my daily drivers for years, heaters still work excellent.
Always living near the great lakes, I usually need a heater core to keep moisture off the inside of the windshield. Even in Phoenix AZ in January it was 40 degrees a few mornings, Florida the same way, in February when I went to the Daytona 500 and stayed in the panhandle for a week, 40 degrees wasn't unusual.
If you're not going to store a car from October to March you'll need a heater core at sometime. When we go to dirt tracks even during summer months I need a heater to keep moisture off the windshield after the sun goes down.
