Unused RAM is wasted RAM.
Android, like Mac OS and Ubuntu, wants to use all the RAM it can, because that's how it was designed to work. There are settings (in Android we call them "minfree" settings) to tell the system how much RAM to leave free and available, but the rest is designed to fill up as fast as possible and stay that way.
You're probably thinking "What's it filling up with?" That's a great question. After the system, graphics, radios, and any other tweaks to RAM are done loading, the rest is there to load apps
into memory, right up to the point where the OS says to stop. Load the app as it's being used, and keep it there for the next time until it needs flushed to free space for something else. The more you use the system, the better it gets at keeping the right things loaded and ready to go. Think about how you use your phone -- you might have 100 apps
installed, but there are a few you always are opening and using. Eventually, most of those apps
will be stored in your RAM, simply because you're always opening them and loading them into the RAM if they weren't already there -- and "erasing" other apps
that were there first. Loading an app from your storage takes longer, is harder on the battery, and overall worse than loading it from it's cached position in RAM.
Consider this -- Jerry did/said/thought something that made his wife mad (yes, she can read my thoughts), so he bought flowers from the 7-Eleven and wants to make a mix CD of her favorite Rod Stewart songs to give to her and get his *** out of the doghouse. It could happen. Consider which is more efficient:
Burn 20 songs to a CD, give to wife, and smile while she plays it.
Burn one song to a CD, let her listen, then erase it and burn the next song.
That's what your phone (or tablet) has to consider. Loading Google Talk to RAM once, and having it there to open almost instantly is far better than loading it each and every time you want to use it. So why kill it off? It's not like you'll never use it again, and nothing else is going to use that RAM while it's sitting empty -- at that point, it's wasted space. You will also use a lot more battery power re-opening Talk every time you get a message than you will by having the zeros held as ones on your RAM. The folks who built Android really did know what they were doing when it comes to memory management. After the parameters are set, and the amount the OS can use to "swap" for it's normal operations, the rest is simply wasted if we're not using it. What is cached in RAM is just sitting there, not using any CPU cycles, but ready to get pushed to the front and appear on the screen as fast as it can, and not use the extra battery needed to start it up from disk again.