Not really. At least not with (the classic) Mac OS. The memory model is designed in a way that software does a lot of its own memory management because it is probable that the OS will tell it that it is out of memory. I think that this created a situation where developers created much more memory conscious applications. There are, of course, exceptions to that rule. I would imagine that Photoshop is a heck of a lot more efficient while dealing with large images if it doesn't have to get creative with memory management on its own. But that isn't run of the mill stuff.
Modern operating systems do tend to benefit from extra memory. Not only because they are memory gluttons to start with, but because they do interesting things -- like cache the file system. (Well Linux does, and I seem to recall hearing that Vista does. Mac OS X, I can't check.) Of course, software is also a lot less conscious of how it allocates memory too, because software developers know that modern virtual memory implementations are fairly efficient. That being said, you still have to deal with diminishing returns.
That being said, RAM is relatively cheap these days. So, I guess, why not?