There is a criticism going around that New Kadampa Tradition (NKT) students are not allowed to attend teachings from other Buddhist teachers. This is not true. They have freedom.
It is true that NKT students, if they have chosen their spiritual path, are encouraged not to mix traditions together but instead to emphasize their own practices and teachings, understanding them to present a complete path to enlightenment. However, this advice is given in the context of non-sectarianism, respecting other traditions and seeing them as also having a complete path to liberation or enlightenment. Those attending NKT classes are not told that they listen only to NKT teachers.
For example, one person wrote in early 2009:
I have practiced at both the FPMT and the NKT for over two years. Throughout that time I was in FP. My NKT Centre has known this and I’ve had frank discussions about it with all involved, including the Resident Teacher. It’s never been an issue. They have also known I took my refuge vows with the FPMT. My teacher even offered to start calling me by my refuge name (yes, you guessed it – it doesn’t have Kelsang in it). I’ve also told them that I have attended classes at the Tibetan Buddhist Society (another Gelug organisation), at Diamond Way (a Karma Kagyu school) and at The Institute of Tibetan Healing Practices (another Gelug-based group).
The NKT no longer typically invites Buddhist teachers to teach at NKT Centers. This is not out of disrespect for these teachers. In Great Treasury of Merit, Geshe Kelsang explains how it is necessary to rely upon one teacher and one tradition to attain results:
Experience shows that realizations come from deep, unchanging faith and that this faith comes from following one tradition purely – relying upon one teacher, practising only his teachings, and following his Dharma Protector. If we mix traditions many obstacles arise and it takes a long time for us to attain realizations.
This is, however, simply advice to help students make quick spiritual progress. In practice, NKT students are free to do what they want.