I suspect the problem Google has with a lot of things is scale. To take this for example it might not seem like a big deal to pre-cache images but when you look at the millions of people who probably use Google maps and assume that a lot of that cache will be wasted (people closing the site, people only going 1 of possible 3 ways at an intersection, etc...)

So even though each individual is only caching a few wasted images per visit the end result is probably terrabytes of wasted bandwidth for something that's a fairly minor feature (I agree it would be nice but it's not a "must have")