It is true that Intuition is of ancillary importance in the case of a dominant Thinking type, which you call the ITJ.
The difference between ISTJ (Ti-Se) and INTJ (Ti-Ne) is that in the case of the former Intuition is used to support concrete, sensorial observations. That is exactly the case in the scenario of a mechanic that I have mentioned earlier.
In the case of the INTJ, Intuition is used directly to support logical analysis. That is what I do, I do look for new possibilities and ideas (otherwise I would not be interested in theory) in order to find a way to properly analyze subjects that I am interested in.
Thus, the main difference between the ISTJ and the INTJ is that the former tends not to be much interested in abstract ideas, yet the latter is.
If an ISTJ (Ti-Se) was a philosopher or a scientist, he would not want to deal with abstractions like typology. Instead he would want to concern himself with ideas that one could study without heavy reliance on imagination. For example, he would want to rely very heavily on empirical investigation, he would want to avoid complex chains of reasoning, as they require a heavy reliance on imagination.
I rely very heavily on complex chains of reasoning.