On another occasion, I recall an instructor who would never let a solo student practice stalls in the practice area. It's hard to imagine his students becoming confident about stall recognition and recovery unless those students are willing and able to do the deed on their own.
The point of solo is not to give the instructor a rest from the typical near-death experiences he or she often experiences during training (I jest, of course). Instead, it's to help the student develop confidence. Given that the private certificate calls for less minimum solo time than it once did, it's now more important than ever to let the student leave the nest without sending a baby monitor along. I recently heard of another instructor who requires his students to check with him via cell phone when arriving at each cross-country location. That's a tad extreme, in my book.
If you feel confident enough to let your students take an airplane cross-country alone, then at least let them earn the confidence they deserve to earn by doing it alone. Sure, if they need to call or check in, they'll do it. But we diminish students' developing self-reliance and self-confidence when we don't trust them to make the right decisions on their own.