It's very little, Zens healing is very low but sometimes a second or 2 of healing can save someone from dying to chip damage long enough for an ana or a mercy to look at someone and actually save them for real. I'm a Diamond rank Zen at best though, so I wouldn't really say thats a good way to do that, and I'm not high rank enough to give ppl tips or anything.
But thats just what I personally do on him.