from my comments under the topic of "modifying the form"
seconding Kalamondin's comments, i would like to answer your question with another question, just exactly what are the bennefits of yang style tai chi? anyone who has read about the boxer rebellion or heard their master speak about guns, not just automatic, but even the revolver kind, know that physical self defense is probably the lowest and least important benefit you can gain from tai chi. some here may disagree with me, but in my own opinion, modification of the form will not effect the possible benefits of tai chi at all. i have had two different tai chi teachers, my first taught a mixed form with a lot of shaolin praying mantis in it. i later switched to yang family style which i liked much better because the stances are far more natural(read here soft and fluidic) than the shaolin ones. my point is this, even though the forms were quite different, i witnessed that both styles and both advanced students of each art possessed nearly the same abilities when it came to self defense and chi cultivation such as fa jing. in my own opinion, i think the forms are given far to much importance. the forms are essentially there to teach you how to combine the original 8 gates and learn how to work with chi properly. the forms are actually artificial and when you gain true ability, your actual movements in a given situation will be quite different. in terms of self defense, your movements, if natural, will be a complete bastardization of the form. in my opinion, there is far to little discussion about the topic of the chi gong and nei gong work that should be accompanied by the forms training. here is the real bedrock of tai chi, and with out it, you will only be practicing chuan, and not tai chi chuan. i would also add that if you watch any two advanced students of the same school or teacher, you will notice that their forms are not exact. this is because no one, no matter how similar they may be physically speaking to another person, internally everyone is quite different and will have external manifestations in form because of this. so to finish up with my comment, i will say this, try to learn the forms as best you can, it will help you and your teacher to teach you the art. however, in the long run the forms will only become a kind of exercise or meditation and the only real use you will have for the forms is the same use your teacher has now, helping new students along path of learning tai chi chuan.