I often see newer people staring on their knees. There two primary reasons not to do this; higher risk of injury, and it's not a realistic fighting scenario. As to the second reason, here a few of resources on that.
The first is a video of Jay discussing that issue:
The second is an article from Stephan Kesting, who supports what Jay says on the topic:
First of all, starting on the knees has no application to modern self defense.
This might not always have been true. In medieval Japan, after all, people spent a lot of time on their knees, and I’m sure that people did get attacked while kneeling. That’s why many traditional Japanese jujutsu systems include armed and empty handed techniques for when both people are kneeling. But this kind of scenario – two people kneeling in front of each other – in today’s day and age is exceedingly rare!
Read his entire article here.