You are all wonderful, friends. Frankly, I know Turkish but I wouldn't have been able to describe the subject as well as you if I had been asked.
As you have already stated, "Ali süt içer" refers to Ali's general preference in life. For example, in my family, everyone prefers a different kind of drink for breakfast. I like tea, my sister likes coffee and Ali drinks milk.
"Ali sütü içer" refers to particular milk which is known by the speaker and the listener at a particular time. I.e. Ali drinks the milk.
You can ask me for examples and excuse me if I can't describe the grammar well.