I have to agree; if you have classes in which newcomers cannot safely participate, then you should close those classes to newcomers. One of the "rougher" dojo I've visited did just that: an intro class in which I was encouraged to participate, and an advanced class that would have beat the tar out of me, so they asked me to watch instead.
Even there, sensei spent some time with the most junior of the advanced students, making sure his ukemi were up to the challenge.
My home dojo, which is small, has only a brief beginners' course and then open classes. The last session of the beginners' course is spent teaching some basic safety skills for the open classes: how to pull out of a throw (with actual practice), which drills shouldn't be tried at full speed at first, etc.
This allows the open classes to be taught at a faster pace than intro ones, while giving new people some tools for protecting themselves. I think it's important not to put social pressure on newcomers to practice above their skill level, or even let them imagine such pressure. The explicit training session makes it very clear that going slow, sitting out, etc. is appropriate.
I'm 40 and a klutz and in 14 months of practice the only one who's hurt me at all seriously was me (wrenched neck) and that didn't seem preventable. (In a moment of stupidity, I decided that if the back roll wasn't working, I could put some momentum into it and force it to work. Ow.) Some bruises, tweaked wrists, a few breakfalls I wasn't quite expecting, but I have not felt endangered.
My father gave up aikido after a serious neck injury that still pains him a decade later. He told me that story over and over as a not-too-subtle way of dissuading me from taking it up, but I was given courage by my sense that my dojomates were looking out for me and would do their level best not to hurt me. Without that I think I would have been frightened away, which would have been a pity.