It's not uncommon. For the following reasons

1. If the student is already staying and working at school (say working at lab) during the summer, taking 1-2 summer courses is not a bad idea. Tuition in summer is typically much cheaper. 1 course could be $3,000-3,500. During school year, it is almost $7,000 per course on average at top private schools. If a kid takes 2-3 courses during a few summers, he can easily graduate one semester or even one year earlier. That's a saving of $35,000-70,000.

2. Some courses may need pre-requisite. Taking a pre-requisite during the summer will allow the student to register for a specific course during regular sessions - especially when some key course is offered only once a year instead of once per term.

3. You could register for enriching courses, such as arts, foreign language, PE, etc to fulfill degree requirements, or just for fun.

Again, if you are already spending $70K/yr for ~8 months, spending an extra $5k isn't going to kill you, and certainly not a wild idea.

But only if student has a legit reason to stay at school. Otherwise, travel! Taking internship! ... should be time better spent - or try mixing & matching and having them all.