The purpose of glow plugs is to assist starting, or make starting easier. If you've been running the tractor for an hour in the summer, and shut it off to take a 5 minute phone call...would you guys really hold the glow plugs on again for a few seconds? Nothing to gain...if the engine is already warmed up, even on a cold day...just restart the engine.
My B2920 starts perfectly fine without using the glow plugs during the warmer weather. When it gets cold, if you try to start it, it will crank much longer and start running rougher.
Using the glow plugs isn't "easier" for your battery, either. The glow plugs have a tremendous electrical draw. So if you can start without using pre-heat, that is actually easier for the battery unless the engine cranks excessively to start. The more you use your glow plugs, they also start to wear out. It will take a long time, but using them less extends their life expectancy.
If you have a modern emission-controlled DPF tractor, it is probably in your best interest to let the automatic timer turn off before starting.