Essentially what this does is makes sure your RAM is working correctly. The more RAM you have, the longer this test takes b/c the OS is testing every component of the RAM to make sure it's within specifications.
If you've been running this entire time with the test on, I don't think you are opening yourself up to any risk. It's very rare for a RAM module to "go bad". They're usually either bad from the start, or work well for a very long time.
They don't make it easy (well, that opinion is relative, I guess) b/c a lot of the issues with OS 9 and earlier can be attributed to bad ram, or RAM that's outside Apple's specifications. That's why a few months ago, with the release of a firmware update, Apple "tightened the reins" on RAM manufacturers to enforce their specificaitons (I'm going to ignore whether or not this was done in a correct fashion).
And if you add more RAM, you can always turn the test back on temporarily to have it tested.
I don't think it's really a big deal at this point to turn it off.