We use feature/topic branches that we then merge to a main branch. It would be useful to trigger a Pipelines build when a Pull Request is created. The alternative at the moment is to trigger builds on a branch name pattern. This however has some disadvantages:

• Every commit on feature branches would trigger a build, we don't expect them to be all stable commits so this will just waste build minutes
• We'd need to enforce a branch naming convention (which might be a good thing in general but would be really annoying if your branch is not building because you misspelled something)

We've used PR triggers in other CI services and found them quite useful. I would suggest Pipelines having something like this:

#!yml

pipelines:
  branches:
    master
      - step:
  pull-request-into:
    master
       - step:
           ...

This way any PR into the specified branch will trigger a build when you create/update a PR