Well, I'm going to go against the grain and say go for it. It's soap for gosh sakes. It doesn't go bad and it's not any more unsanitary than using the same bar of soap in the shower for more than one day. I never dump out my soap. I use one bowl to hold the soap and another to build the lather. Any residue left in my brush after shaving is scraped on the rim of the lather bowl before I wash out the brush. The remaining lather in the lather bowl is just left for the next time. If there is any significant accumulation in the lather bowl, I don't bother getting any soap from the soap bowl. I just build my lather from the residue in the lather bowl.

Some think that's disgusting. If you're one of them, then don't do it. On the other hand, some people in this forum report that their soap lasts for about a month while I have never had one last less than 5 years without ever shortchanging myself on lather. Take your pick which way you want to go.