We run our XP on 'trap' in an attempt to hold the sensitivity steady until the end of the filament's life. You can tell when a filament is getting old by looking at the box/trap ratio and seeing that it is drifting to one side and losing sensitivity most likely due to tungsten sag. So, when running on 'total' you typically get a few weeks notice until it dies. If you run on 'trap' then you'll get about 3 days notice if you're lucky, but the sensitivity should be the same, and the box/trap ratio will drift to one side, then die.
Here's where it gets tricky if you choose to do this: Isodat does not control the current to keep the mA at 1.5, rather, you do manually by adjusting the emission. Also, the software does not reset to read as it should, therefore caution is advised. Always start with the emission at zero after you switch to 'trap' and turn the source on (emission now becomes represented by trap current only)! As you slowly ramp up you'll notice that emission is reading only trap current, box will follow. I always run south of 1.5mA so that when the filament tries to hold the trap current as it's dying it won't go too far over the 1.5mA total. If you forget to do this and leave emission on 1.5mA when you switch the source on I'm almost positive you'll blow the filament because you'll be closer to 3mA.
If I'm leaving anything out I'm sure others have more to add. Hope this helps.