Lineage is important. It is difficult to understand when there are so many people who claim to be experts in tai chi (or any other martial art, for that matter). Part of the "mysterious transmission" is given from master to disciple/student, this can only be accomplished through "lineage" and hands on training. Without it, it might nearly be impossible to attain extremely high level of skill.
This "mysterious transmission" can be sensitivity training, energy flow and timing training, special techniques, and penetrating power training, to name just a few aspects. In many of todays western schools, it sounds a little rediculous. But, from my training/teaching experience, lineage is important. Without it, maybe if you're really, really intelligent, lucky, and work 8-12 hours a day on developing skill (especially internal) you'll figure some of it out.
Any way. My short 2 cents.
Martin Yoder