[Ferrotoral]

There are two aspects to it:
1) Tassawuf is a science,you want to learn it,you need a teacher...and obviously you need to obey him as for as the subject under discussion is concerned.This blind obedience does not stretch to any other aspect of your life,unless it is somehow concerned with the subject under discussion.For example if you are not earning Rizq e halal your Sufi teacher has every right to put you back on the right track etc.
2)Blind obedience to Shaikh in all the matters,considering his halal as halal...and his haram as haram.....without checking it with Quran and Sunna.This kind of bayat is haram,null and void and one must leave such Shaikh as quickly as possible.
As for the question of 'Bayat per se"', I don,t see any reason why it should be haram.A student comes to a teacher,make tauba from the mistakes of his previous life,and make a pledge that he will spend the rest of his life according to Quran and Sunna,under the guidance of the same spiritual master.This can only be called good,and there is evidence from the life of our Prophet sallaho alai he wa sallam,taking bayat from different people for different reasons,like they will not commit zina,or steal etc.