Hey Raymonde,
I have been using the VSL expression maps for a very long time and I think they are really good and well thought-through. They cover a lot of groud and satisfied most of my needs in the past.
Quite recently, I came to the conclusion that in order to speed up my workflow and be more flexible, I need to have expression maps that are specifically tailored to my way of working. So I agree, sooner or later you might want to think about creating your own expression maps.
Is it worth it? Yes.
Is it really necessary? Probably, but maybe not. The VSL expression maps are damn good (kudos to the VSL team).
My specific reasonst to go with cusom expression maps were:
For one, I hate attributes and do everything with directions. I have accidently deleted so many midi notes by deleting attributes in the expression map secion, I simply figured that attributes do not work for me personally.
And for another, I wanted an expression map that allows me to copy midi from one track to any other track (percussion excluded of course) ideally without the need to change anything. And still have enough space to setup custom articulations when a piece calls for them.
In summary I think that if you want to make the most out of your time, then you probably want to create your own custom expression maps.
Hermann