Sight gauge on MLT?

Is there a point to this because wont the grains skew the reading?
Won't the grains clog it?

It won't give you an accurate volume once the grains are added without doing some calculations on grain absortion but, it is also useful for other reasons. For example, to see strike water volume before adding grains, to see how much is left in the mlt when you are draining, or for keeping a certain level when fly sparging.

The dip tube for it should be installed under the false bottom to keep grains from entering it, or you could install a separate bazooka screen on it if that's what your setup uses.

The volume should be read by what has moved out of the hlt less volume below the false bottom.

With the sight glass I can use a rubber gasket to mark where vol should be when fly sparging or my fill level

I just use my foot-long lab thermometer...I do small brews but it should work for a 5-gallon batch...just fill it once with known volumes and mark the "temp" at which the water level is what you are after...its fairly accurate...you can even measure how much 5 or 10 degrees is in your MLT so you know if you are a pint, quart or whatever over or under