I see nothing wrong with beginners practicing together. This is aikido, the art that can be practiced until your long in the tooth, so you got time to fix any problems. I haven't suffered from it. Besides, what's an instructor to do when starting up a new dojo.
In a new dojo it is excusable as an unavoidable necessity. Once you have some trained people though, I really can't see any excuse for inflicting substandard training on your students. Particularly when that training can teach them bad habits that may take years to unlearn.
We want our students to make as much progress as possible, so loading them down with the weight of easily avoided bad habits seems counterproductive.