If I were a pro in a race with wheel support and I wasn't planning on fixing the flat myself (carrying a spare tubular or a spare tube for a clincher), I would absolutely use tubulars.

When a tubular goes flat, you can still ride on it. The tire is glued to the rim, so unlike a flat clincher where the beads are no longer locked in, the tire won't roll off while riding in a straight line. Yes, you could damage the rim if you hit a hole or rock, and you probably need to slow down some, or not lean hard into any corners...but you can signal for neutral support without having to come to a complete stop right away. This could mean seconds...or minutes saved depending how far back wheel support is. Or...if you're only a couple miles from the finish...just ride the flat tubular in rather than stopping.

You see it quite often in pro cycling. A guy has a flat, puts his hand up, and keeps riding until either a teammate or the team car is ready to give them a wheel...and only then do they stop.