It's not time that's the issue, it's the opportunity to boost the self-confidence of the stude and to start developing captaincy.
Yes there's a small risk, but it's significantly less than sending people on their first solo and IMHO the benefit outweighs it. If we are never to trust a stude to be alone in an aircraft with the engine running then the first time they are alone is on that first solo, and the first time they start or refuel by themselves is when they're on their first un-checked solo circuits - that's too big a step.
Students will perceive it to be OK to enter/exit an airplane with the engine running..a dangerous habit, that was one of my points.
Aeronautical decision making skills are the hardest to learn and also the hardest to teach.
My other point is more academic, if it is to be a dual flight the instructor is responsible for everything,a proper preflight and start-up included.
Therefore the instructor should be present.
On a solo flight there is always an increased risk, inherent to flight training.
To minimize that risk we should be adhereing to safe practices during dual flights, at least to give the right example.