The need for a teacher I would say varies by methods, tradition, your own propensities and how far you want to go. Some periods of your life could greatly benefit from it, others it is so and so. And Buddhism, Mahayana Buddhism at any rate, is also very much about unearthing the inner teacher, though of course this also requires learning not only to access that, but also to sift that from our own delusion. As the late Hsuan Hua said - until you are either an arhat or on the irreversible bhumi, you can never truly trust your own thinking.
That said, Huineng didn't need a teacher to have a deep awakening and be the most profound guy in the fifth ancestor's monastery. He did need a teacher to go from there to full awakening. Bodhidharma had a teacher who gave him transmission. He didn't need one for the nine years he sat facing a wall though. Neither insisted on the necessity of a teacher.
On top of that, there are formless teachers, sometimes aiding unseen as well. For the same reason, I wouldn't be rolling my head laughing at the notion that Amitabha is all the teacher you need for pureland practise.
I think the question is not formed rightly though. Rather than ask "is a teacher necessary" we should be asking "how useful is a teacher?" If we are sincere and genuinely interested in developing the path in a way that goes above our own preferences and bias (a highly necessary prerequisite for one who intends to go at it on his or her own), then really we should be looking for all the best aides and supports for practise we can find.
Seated meditation, for example, may not be a necessary requisite for awakening in the strictest sense but there is little doubt it makes a huge positive difference to one's cultivation that serious practitioners would not want to be without.
Similarly, I would say the benefits of finding a teacher are too good to want to miss out on. They help with a host of supports to the storm of spiritual qualities cultivators need to kick off to get on the track of genuine awakening. So we see, though Bodhidharma does not insist on the necessity of a teacher for awakening, he does make the point that to get there without one requires a truly exceptional storm of conditions:
Only one person in a million becomes enlightened without a teacher’s help. If, though, by the conjunction of conditions, someone understands what the Buddha meant, that person doesn’t need a teacher. Such a person has a natural awareness superior to anything taught. But unless you’re so blessed, study hard, and by means of instruction you’ll understand.
One of the main benefits of a teacher to me is not so much what they say as it is what they do. The conduct and demeanour of deeply realised beings sets a totally different standard of manifesting practise than we can imagine for ourselves. What we unconsciously pick up from such association goes a long way towards acting as a kind of inner compass for adjusting the strings of practise into a harmonious chord that addresses the whole range of practise.
That and the ability of such adepts to speak turning phrases. Not necessarily of the '"Katz!" -> Boom! Awakening!" variety, but of speaking some words that are able to strike a powerful and transformative chord within ourselves because they are heard not just at the right time for us, but come from a place of deeply cultivated inner power, insight and sincerity on behalf of the teacher that enable such words to strike such a clean note that can reverberate to our very core. For many years, I had never been able to sit for longer than 20 minutes or so, 30 minutes at a stretch (but not a pleasant one) for a variety of reasons. Then I heard a true adept say that if I just learned the art of sitting in meditation for an hour a day then, no matter what else happens, he promised one's life would be highly fulfilling in the end. That rang home as such an obvious truth coming from a guy like him and somehow all my obstacles evaporated after that to the point that I was sitting 60+ minutes sessions comfortably every day four months later.