Instead, I see a lot of people thinking that they have to fulfill a specific role throughout a match as if this were Heroes of the Storm (or some other MOBA) or speak of "main" and "alt" Heroes as if this were World of Warcraft. That's not the correct approach to Overwatch.
Heroes have distinct suites of abilities. Accurately reading the tactical situation, and then swapping to the Hero that best meets that situation, is something that players are expected to do to win consistently.
Starting with one team composition and stubbornly insisting upon it will lose to a team composition that flexes along with the evolving gameplay state. That is why this Beta has a native voice chat client (ala Destiny), so groups can coordinate in real-time using the most efficient means (voice) at hand.
As the Beta goes forward, and successive waves of players join the fun, I expect this to become the best practice that the community promotes once the idea achieves critical mass and becomes widely accepted.
(Note: This was meant to be posted on this date, but due to an unforseen access outage, it was not posted until the 3rd of November, 2015.)