I don't think it means much if you lump eveyone together in "what do
the pros use". Obviously, some people will use tubulars, others
clinchers. There is no "standard."
The immutable facts are clear. Tubulars ride nicer and the wheels are
tougher. Clinchers allow a more compact emergency kit, and a fully
functional tyre after an on the road flat repair (tubulars cannot take
a hard corner after an on the road fix).
Clincher riders who carry only a tube will eventually suffer a carcass
failure or sidewall blowout and have to walk home, while the guys with
tubulars will have a complete spare tyre, as will the guys that carry a
foldable. (I know this from painful experience).
Clinchers allow you to ride through trashy streets, get flats,
patch/replace the tubes over and over, and still get 2000 miles out of
the tyre. Tubulars, except for TUFO, do not allow this behavior. (TUFOS
can be made self-sealing and so will literally seal the hole the moment
you ride over a piece of glass).
If you are racing the criterium in Hunting Park you aught to ride
clinchers or TUFOS rather than your $70 silks. Just in case.
If you are in the habit of riding over whatever the road throws in
front of you, and/or believe Jobst when he says that "wiping tyres
makes no difference" then you probably aught to copy all the fred and
nancies and ride a set of deep-dish cosmic pizza wheels with michelin
marshmellow tyres and a couple spare tubes and foldies and those stupid
compressed air thingies instead of a real pump 8-0
But if you are a cool dude, you will ride a real wheel with real tyres
and you might even look where you are going :-)