If you have a very consistent and reliable source of reviews via in person requests, by all means, keep it up! Most of the time though, even the most well-meaning patient gets distracted and forgets to leave the review by the time they get home. By having the requests sent to their cell phones or email inboxes, they get the additional reminders and are more likely to leave the review.
It will also offer the option for the patient to contact the office if they express that they were unhappy with their visit. This will hopefully let them feel like they're being taken care of, reducing the likelihood that they leave the review in a public forum.