Even if the boot time is fast, you lose a lot of the program states. Not only it takes extra time to load those applications, it's also a fair amount of effort to put everything back where it should be.
If it was necessary to shut computers down, no problem, it's not too much time and effort. But there's normally no need to shut computers down, it's just wasted time with no benefits (usually).
Even if it's only one app, what is the purpose? To save on electricity that powers RAM?