The more classes you have, the more diversity and options your team will bring to the table. Also, the more you have, the further from optimal efficiency you will get.

For PvE, an entirely mixed group is very viable. For PvP, its doable, but not at a high level of play.

With 5x Shaman's or 5x Hunters, you can have the same keys in the same places, and click them at once. You'll lose some efficiency because of the limits of follow and assist etc, but they'll be close to optimal.

With different classes, you lose more then with mostly the same. You're still able to do an amazing amount with them. Via the Click Castsequence method, you can have toons cast spells with variable cooldowns, virtually as they cooldown, by tweaking your macro's. The example showed a Druid who, via one button spam, casts Insect Swarm every 18 seconds, Moonfire every 12 seconds, and Wrath spam when not doing one of the above two timed spells.

The more classes you have, the more complex your macro's will become. Still, the combination of tweaking macro's and your settings to get things to work properly combined with beating content by yourself can be very satisfying.

I wouldn't go five different classes for the sake of having five classes. Just like you don't go 4x Shaman plus 1x Paladin because that is one of the easiest options to go. You should choose classes which you will enjoy playing.