I've spent many hours at the repair bench early in my career. As other technicians will tell you, while a regulator that has been completely flooded to the point that internal metal parts are beyond cleaning is rare, the majority of regulators have some degree of water intrusion. Completely dunking your first stage without the dust cap in place is like doing a giant stride with a snorkel in your mouth - usually something you'll do once if ever in your diving career. The main sources of water intrusion are between dives - either during the process of switching tanks (we've all seen people using air pressure to blow water "off" thir dust cap and inlet valve) or while divemasters are topping off your tanks (how many times have you seen first stages left off the valve waiting for a fill without the dust cap in place?) or during the rinsing process. Unless you rinse your first stage while it's pressurized, there is a chance that water is going to creep past even a properly installed dust cap.
While the replacement of metal parts and water traveling through the system into pressure gauges and air integrated computers is a financial consideration, as I said earlier it is unusual to find that kind of damage. The more important issue that DVT addresses is performance. Water entering the first stage has a number of affects - first, corrosion builds up on and in the inlet filter, ultimately reducing air flow. Second, and most common is damage to lubricants and o-rings. Years ago we had Nitrox specific regs where we used Christo-Lube, while on the standard air regs we used silicone grease. We found that the Nitrox products consistently out-performed the standard products - solely due to the lubricant used. Performance is impacted significantly when lubricants and/or o-rings are compromised.
