Rob hit most all the high points. The only thing keeping metal from touching metal in the bearings is the oil pressure. The rod journals on the crankshaft are the last place in the engine to see oil pressure at startup. Unfortunatly, they are also the ones under the most load as they force the rod/piston up to full compression. By removing the compression load and allowing the oil pressure to build during the first few seconds of engine cranking(pre-lube), you keep the rod bearings from comming under heavy load with no oil pressure and allowing the bearing surfaces to possibly touch.
Another potential issue is hydraulic lock. The starter motor is very high torque. The engine is very high compression. A very small ammount of fluid in a cylinder could cause the piston to be unable to complete a cycle because of the incompressability of fluid. If this happens, that spinning energy in the starter, crank and flywheel has to be absorbed somewhere. It can bend rods, twist crankshafts, blow headgaskets, fracture pistons and crack heads. On the marine diesels I have operated, we had to do whats called a "blow-down" prior to start if the engine had not been run in the last 4 hours. A blowdown is basically cranking the engine with compression relieved to purge any fluids that may have found their way into the cylinder from leaking injectors, leaking head gaskets, cracked heads or even water from the exhaust or intake. All of those are bad, but not as bad as having to change out a half a ton of crankshaft that has been twisted
Electric starter motors draw more current and make more heat at lower RPM's than at higher getting an engine up to speed. Compression release allows the starter to accelerate to a more efficient higher speed faster, particularly in colder weather.
The way I figure, they put it on there for a reason. In my opinion, it is better to use it than not to use it so I might as well. In my shipboard machinery experience, I have never had a marine diesel that I operated that we did not "blow down" and use pre-lube prior to startup. I always use the compression release.
My routine startup procedure warm or cold is as follows:
1. Advance hand throttle slightly above normal idle position.
2. Turn key to glowplug/preheat position for about 20 seconds or untill I see the amp gauge come off the -30 amp peg(I skip this step if I am restarting a warm engine)
3. Turn and hold decompression lever.
4. While holding lever, turn key to start and crank engine till I see the oil
pressure needle start to move(about 2-3 seconds on my 284).
5. As soon as I see the needle move, I drop the decompression lever followed quickly by the starter key. The engine starts as you drop the lever and compression is restored. As the engine RPM increases, I reduce the hand throttle to not let the RPM exceed 900 IDLE. Then I let her warm up a bit before I start work.