Learning is a holistic process that involves one's intellect, emotion, and volition. While other people's differing perspectives may benefit us by challenging what we've taken for granted, learning from those whose ideas who share may be more effective.
To begin with, contradiction from others may cause emotional filters that would inhibit learning. Examples abound as to how stress caused by disagreement would inhibit learning. Take the conflicts in the early years in the Chinese Communist Party for example. Miao did not learn from the differing viewpoints offered by his comrades, but rather he refused their viewpoints and even annihilated them. Their disagreement, rather than provide an open forum for learning, turned into a struggle to take sides, for very few circumstances would let people maintain their absolute neutrality and not to take things personally. Different viewpoints are often intermingled with personal interest, identity, and loyalty to one's team members. Theoretically, learning from dissenters might work, but not technically. As well illustrated in the movie "The King's Speech." George VI's speech impediment is caused by ill-treatment of his nanny and overachieving but mischievous brother. But his speech therapist, by helping him overcome all the unnecessary and even imagined stress, help him overcome his seeming inability to addresss a public audience, and successfully delivered many inspiring speeches to his people to resist the German bombing.
On the other hand, affinity in heart and mind in a team is the most conducive element for achieving mutual learning, and eventually success. For a learner during his formative years, he needs an environment in which he would have many like-minded and kind people to help him. In the 2007 award-winning movie, "Amazing Grace," William Wiberforce and his like-minded and like-hearted colleagues work together to achieve the humanly impossible task, the ceasing of the slave trade. Wiberfore's ability to overcome intimidating opponents in the parliament is not possible without the back-up of his peers. His posture in the parliament, however, is not a learning one, but a guarding one. 
Admittedly, one has a lot to learn from his/her opponents. Yet what one's opponents contribute is mostly to raise one's adversity quocient, not one's intelligence. Opposition may even harm one's emotional well-being.
In conclusion, one learns the best from like-minded comrades, not inimical opponents.