Mostly because I got used with that feature on ProEngineer.
Can be used for eveything...
I play a lot with the mirror tool, for example. Even knowing that the normal line serves very well.
It's more a psychologic need... That infinite line feels more solid... more concrete... It is like the main pillar of a building, where all the other measures refer to. I like that proEngineer strategy of making references of everything. A vertical and horizontal infinite line has been always the first thing I put in the sketch.
I recognize that in this case this line would indeed be redundant.
I liked your Idea of the axis. Resolves 90% of the problem. Too bad it cannot be turned, angled, etc without leaving the sketch edit. line is also easier to create than an axis.
Will use a normal line until someone come with something more specific.
So far, thanks for the solution.