If you need proof, go to the NTSB aviation accident site and search for
midair crashes. Two notable accidents that come immediately to my mind
since I began flying in March of 2002 are the fatal MAC at Caldwell, NJ
and Carlsbad, CA.

My friend was the pilot of the Mooney at the Carlsbad accident. I keep
track of that NTSB report and now a year and a half later it is still
not closed and marked as preliminary. In that prelim report, apparently
the tower told both pilots that a plane is headed in their direction.
So a tower can't prevent any collision but they certainly help the
organization of that traffic.

One time in the pattern at SQL (San Carlos, CA), a mooney pilot
which I presume is someone with more than 41 hours total time couldn't
make a radio call to save his life (or anyone elses). I was in the
pattern and he buzzed off. 10 minutes later he comes back was told
to report at a local VFR point. I told my instructor "Oh ****. he's
back." Well sure enough he doesn't repot and buzzes onto final
and almost lands on another plane. He is told to go "climb to 1200
and circle the airport for spacing." Well he climbs to 800 (pattern
altitude), does one circle and buzzes into the pattern. My heart
was racing as I was watching/listening to this happen. when the
accident almost happened, I told my CFI, "you watch that asshole.
I'll keep an eye on the traffic in front of me." Towers won't
prevent anything but they certainly can help.

On my PPL checkride everyone and their mother was out flying (12/17/03).
I did my engine out landing at TCY (Tracy, CA). A 172 (?, a 4-seater
high-wing) was holding short of the runway and despite our radio
calls he positioned himself right on the numbers and held. No radio,
no looking. I was slow enough and far enough out that it did not
pose a major problem although I was maybe 5 seconds from going around
despite my "engine failure." I'm sure the DE would have been cool
with that but still, just unneeded pressure on the checkride
and unneeded excitement on final at any time. Again, a tower would
have helped a lot as they wouldn't have been cleared onto the runway
(well hopefully).

Gerald