Another advantage of using personal pronouns in essays for IELTS is that it makes your opinion very clear.
The examiner will expect to be able to clearly see what your opinion is (in opinion essays). There is a possibity that if you use impersonal language the examiner will not realise it is your opinion.
This is not to say you have to use personal pronouns as you certainly don't, but if you are a lower level writer it makes it very clear what your opinion is so there are no misunderstandings.