Actually, the lower tube serves two functions: first, it holds the dipstick ridgid so it can't get wound up in the crank when the oil gets into a high "windage" situation. We found that out the hard way on a race motor. And secondly, it helps keep the dipstick from leaking at the mateing surface of the upper dipstick tube because the oil can't get thrown up into the dipstick hole in the block and seep past the upper tube.

John