Where traffic volume is concerned, it only takes two aircraft to produce a mid-air in an otherwise empty pattern.
If a student learns in an ATC environment (in my personal experience as an instructor, anyway) they get comfortable with radio work more quickly.
A couple of weeks ago I had the closest call of my life. I was in the back seat of a training glider with a student up front, being towed by an ADS-B equipped Pawnee, right over our airport at about pattern altitude. The uncontrolled private field is marked on the sectional and the TAC, with the CTAF, a glider symbol, and text about intensive glider activity. Legally, a towing operation has right of way over all other powered aircraft excepting only those on fire or worse.
I spotted a Cessna 172 approaching at about our altitude, and heard the tow pilot make two quick advisory calls that went unanswered. We began a turn to improve our visibility (my glider could not maneuver independently on tow). The Cessna passed barely beneath, and between me and the tow plane, closer to me than the length of the tow rope. I could read his N-number and I could clearly hear his engine over the sound of the Pawnee and the air rushing over the glider. I would estimate the separation at perhaps a wing span. The Cessna continued for a few miles, and then came back for a second pass to return to his 20-mile distant home base, by which time I had climbed another thousand feet and released from the tow. No radio calls from the Cessna were ever heard.
All this happened while we were the only traffic at the field, using proper radio protocol with three sets of eyes in our towplane / glider combination and operating ADS-B.
P.S. I obtained the Cessna's ADS-B trace by using his N-number and filed a near-MAC report with the FSDO, who took no action at all.