When you use LTI for assignments, it is only possible to receive a number grade back from LTI. That may be enough for a large number of LTI integrations. However, what about those where it is beneficial to give more detailed feedback and be able to store that in Moodle?
Example: Mahara portfolios. Currently, you can use the "External tool" but then give a grade on the portfolio which is pushed back to Moodle. However, none of the feedback given on the portfolios is displayed in the gradebook. In addition, none of the grading workflows and rich grading options available via the assignment module are available in the "External tool", such as rubrics, assigning different markers etc. It is possible to update the current assignment submission plugin to support such work flows, but I think it would be nicer if a more native option were available to allow using LTI also in the assignment module rather than solely via the limited external tool.