It depends. Students often consider that the summer is a good time to take required courses and "catch up" in the curriculum. It is important to consider a different view. As emphasized elsewhere in this handout, each student's plan and progress is unique. There is no reality to the concept of "catching up". The consideration that others in the past or students in different curricula see four years as the "right time" to meet undergraduate degree requirements, is not workable by today's standards and expectations. It is more useful, practical and beneficial to expect that each student progresses at his/her own rate, earning grades each semester that he/she considers fairly represent their best efforts, with a general sense of moving toward an individually defined academic goal. Sometimes parents, selected well-meaning relatives, advisors and other concerned faculty, as well as the student himself/herself have to be reminded of this alternative view.
There is always a risk for taking courses in the summer. It's similar to the one for enrolling in 21 credits each semester. Afflicting most often the hardest working, best motivated and well-directed students, such a work load and rate may come at the cost of intellectual fatigue, flagging motivation and a growing concern about whether it's worth all the time and effort.
There are, of course, many excellent reasons to take summer courses. There are also many excellent reasons not to. The best plan can only be devised by evaluating the needs of the moment and working closely with the student's academic advisor. The greatest danger lies in making decisions without asking the right kinds of questions first.