Hi Pierre.
I do agree that there really isn't much point in telling the machine to add more water when it could throw a trouble code or just drain off the excess water.
I firmly believe that unless the machine was designed otherwise, an automatic washer should stay completely automatic. Having to manually do anything other than adding to the dispenser defeats the purpose of having one.
You are right on one point, you never really have to worry about a top loader not using enough water. (Unless you just bought a Speed Queen / Huebsch and you want to raise the water level, but this is a one time thing.) Of course, we're making the assumption that one always uses the water level position at the maximum setting all the time.
Let me set the record straight once and for all though. Once I switched to using a front loader as opposed to using my GE Toploader, all of my clothes came out cleaner, the streaks of detergent I couldn't get out of some of my shirts were now gone and it's done an absolutely amazing job with 75 Liters of water as opposed to the 180 Liters of water per load my old Top loader used.
It's not submerging your clothes in water that does the cleaning, it's the agitation action of the clothing. A lot of top loaders suffer from perfect agitation around the agitator but around the edges of the tub, the clothes just sit there and don't do much, especially in a fully loaded or overloaded washer. (Some members here I'm sure would debate that, but that's been my experience.) My old GE Toploader had a big problem with washing 9 pairs of jeans, there was nearly no turnover. Reducing that to 8 pairs helped, but my jeans would just float to the top and just sit there in a mixed load, until the auger would pull them down again.
Nothing in a front loader is spared. Nothing just sits there or floats.
I'm still absolutely amazed at how, even though I can't see the water level in my front loader, my clothes are dripping wet through the glass.
So, the debate rages on, FL vs TL... well, that's a whole other thread.
I will say this, I would stop worrying about adding more water and let the machine do its job. If you want to use more water, then consider buying a machine which will use more water, or has the option of using more water. (ie. A machine with a comforter or towels cycle.)