Why aren't people pulling out Mercy's pistol more often?

So I hear a lot of complaints about Mercy being just a healbot, but from my perspective if you play her like that, you're not using Mercy quite as I imagine she was supposed to be used. I've experimenting with Mercy a lot on QP to see how it goes and I've discovered that I'm being much more helpful to the team by doing this:

• I use my beam to fill critical HP's back up.
• I damage boost anyone who is currently ulting.
• In between these two activites, I'm landing sweet easy headshots.

While playing with supports I've come to realise that I'm usually putting up waaay much more healing than when I play anyone else but my damage output is near nothing. What's the point in keeping the beam engaged when it's not gonna affect the battlefield whatsoever? Getting some picks on squishies on the other hand, is much better, and I seem to do it quite consistently as Mercy can put out a surprising amount of damage.

So I daresay an unpopular opinion: Mercy's kit is not perfect, but well balanced at the moment, we are just not using it to its full potential and creating a massive prejudice against the character as a whole.

Edit: I've made a little math, correct me if I'm wrong, but here's why shooting is better than boosting: Soldier ideally deals 171 damage per second, if you boost him, you're helping with another 52 damage. But, if you shoot instead of boost, you're adding the ideal 100 damage per second that Mercy deals.