I think the issue is implying that because someone fails a course, that in some way indicates that they wouldn't pass the boards or just "aren't cut out for medicine". Many could make the argument that anyone who doesn't get into a US MD school isn't "cut out" for medicine, but lo and behold, there are thousands of DOs and US-IMGs that prove that they can be proficient, and *gasp* even excellent physicians after not getting into a US MD school.
Having a large attrition rate means that the education system at the school is not adequate to teach the material, the school lacks some form of support system for struggling students, the school is accepting too many students it knows can't succeed, or a combination of all three. Most US medical schools have a ton of support for their students. Most go out of there way to make sure a failing student succeeds. Not all medical schools have good remediation policies though, and a student that would succeed at one school may may never get that far at another because of it. Taking NYCOM for example, they have been known in the past to have a pretty harsh policy of fail 2 classes in MS1 and you're dismissed.
On top of that, given the cost of medical schools in the US (and Big 4 Caribbean for that matter), and given that most students use federal loans, I would hope that schools wouldn't go out of their way to create a huge population of highly indebted students with no prospects.
In any case, there are always people, who, for whatever reason, leave medical school. They realize that its not what they want to do, they want to focus on personal issues, etc. That's why the rate is usually 5-10%, but it really shouldn't be much higher than that.
This is why some attrition is normal.
How big is your class size? 10-15 might still be a small percentage if you are talking 250 students/class. We've lost ~8.5% of our starting class (some to repeating the year, some that just left, etc.), but some may have people that were already repeating the year. I doubt that number will grow above 10%, since classically very few people leave 2nd and 4th year, and only a handful might leave 3rd year.