With the Orb of Harmony, it isn't really as useful as the Orb of Discord right now but I still use it with attack heroes because they almost always play on the offense so they can heal while battling other heroes, but usually giving it to defensive heroes is also good on Defense mode since heroes like Junkrat set-up with the mines and trapper, and if they caught they can heal up and that prevents the risk of getting killed or being caught of guard while setting up. Also Zen's ult is also good when the enemies are capturing a point so you can finish them all off with huge amounts of HP, also contesting the point in the process. So ultimately if played right Zen can be very helpful in Attack and Defense.
Honestly aside from the other support heroes Zen can do massive healing and damaging in the process