I think the answer to your question is going to depend on how a person interprets wudu. You can look at it from two perspectives.
Wudu has to be done exactly like it has been prescribed. For example, if a person urinates or defecates then istinja is required. If you just pass wind then no istinja is required and you only need to do wash the hands, mouth, nose, arms, feet and hands again.
The act of wudu and istinja is to enforce self cleanliness and better health. Therefore, performing wudu (rinsing your mouth etc.) is beneficial before bed even though it will break during sleep. When you wake up, wudu makes sense again since during sleep you may have slightly urinated and again the rinsing of the mouth is beneficial.
Therefore, we have to see what the overall intention of wudu is. In some cases, if there is no water, one can use sand or stone and just do the acts of cleaning. Because of this, I will conclude that when a person wants to pray: They need to make a good effort and clean themselves up with whatever amenities are available.
I live in Canada where we have access to water everywhere. Therefore, a Muslim who lives here and does not perform istinja after passing wind, is not making a good effort. But if a person lives in a third world country where water is not always around and they only have enough water to wash their mouth face etc. then it would not be required of them to do istinja just for passing wind.
So in conclusion, it is both symbolic and for purity. It all depends on availability. If water is not available and the person uses sand, it is a reminder to the self the importance of trying and achieving purity and cleanliness.
You ask the below question:
Maybe it cleans the body to a certain extent, but it wouldn't be considered up to par with today's hand washing standards, given our understanding of microorganisms.
That is a very good question. If a person is aware of these standards and knows the importance of it, then how could the person consider him/herself clean if they do not live up to this standard? How could they consider themselves pure and stand and pray in front of God when they already know they could have made a better effort to clean themselves up? Wudu is simply a reminder to everyone of how important cleanliness is. If there is a new standard that we discover, then we have to live up to that standard or else we are not making a good effort.