As i understand it, the basic constructor was a stopgap fix to an issue where the Request Constructor didn't work at all if the standard constructor blueprints were all obsoleted.  The intent was always to come up with a better fix for the long term.  Perhaps they have done that in 1.12.

It seems to me that basic constructor is only taken when the default Construction line has been obsoleted. That is what I do, and then create a new constructor line with another name so your bases request the basic one and you can still produce faster and longer ranged ones if you like. I am playing on the latest patch but didn't seem like anything has changed in the last few versions

One warning: once you start upgrading the standard constructor blueprint, you will not stop. You will find yourself re-designing every ship before you are satisfied...

Very true.

BTW is there a way to force a ship role for a given blueprint? There's the <Role> tag but it seems to not work, or at least it doesn't affect the role of the ship created from the blueprint.