Before level 30, most people aren't going to be checking what cross-class skills you have, and it won't matter that much. A few of them (like Swiftcast from THM) are really nice, but most others don't make a huge difference.

After level 30, if you aren't a WHM, most people aren't going to want you healing their dungeons. Because regardless of what cross-class skills you take, you won't be as effective a healer as a CNJ as you would be as a WHM.