I've heard of people doing this. I know that in an emergency you go for what you can get but what if your teenaged son was the last one in the shower and..... 
I think the bathtub water is better used for toilet flushing, in the event of a water outage.
We have Fairfax county water (exceptionally reliable) and downhill septic flow (just need gravity, which also seems reliable). Our next door neighbors however have an uphill flow, so they rely on electricity to be able to flush. After the derecho they ran an extension cord across the street from a neighbor who didn't lose power so they could stay in the house.
