I completely agree. This is not just a problem with the Defender, and is even worse if you want to take something from the general tree. I find myself never getting to try out the mid and late-game class options. Three possible solutions:
1) Make heroes level up more frequently. Considering how weak heroes feel compared to trained units, this may not be that unrealistic of an idea.
2) Have more, shorter class branches that require certain levels to begin. Break the longer branches up into three or four bubble-length choices, but have the stronger paths require a certain level. This way you can try out multiple paths without having to go all or nothing down one long path.
3) Heavily trim the trees to remove scaling abilities (combine +10 defense, +15 defense, ect into a "+2 defense per level") and anything that seems boring.