There is no reason other than having the confidence in your cooling system and oil level. Diesels have no problem at PTO rpm forever regardless of load. Your engine, just turning the unloaded gen through the PTO has to put out a couple HP. Not to mention the parasitic loads from hydraulic pump and alternator and fan. There is never a no load condition - not that it would matter to the engine as long as operating temperature can be maintained.
I have a pto generator, if I was concerned about the fueling interval I think I would outfit a 55 gallon drum with a 12 volt pump.
Just a small inline auto type that I could switch on for an or so at a time to refill my tank.
I have enough tractor tank to go 24 hours so I just shut down once a day and fill up, when that wasn't practical for me I had a hand pump
on a 55 gallon barrel I'd fill from my bulk tank with it in the loader bucket or the back of the pickup,
pull up next to the tractor and refill it.
On my Oliver I had marked the dipstick with a full when running mark so I could check the oil level without shutting down.
Would a small generator be less expensive to run at night with lower power demand, probably, do I want to shut down repeatedly
to switch back and forth, NO.
Can a person get by with less then whole house, certainly, but why.
It is up to the individual and their situation to decide how much power and what method to acquire that power.
The ones that can get by with a 2000 watt inverter and want to good for them.
It would not work for me, large house, deep well etc.
On top of that as has been mentioned who knows what else is going on while the power is out.
I can and have worked in my shop including using the welder during a power outage.
I have enough generator (25 kw) that my generated power is cleaner with less dips then
Ni-Mo (National Grid ) provides me.
The last time I was tractor shopping a 540 economy was on my list of desired options,
I have also used adapters to run my 540 on a 1000 rpm output, the success of that
depends on the tractor and the ability of its governor some can do quite well some can't depending on the loads.
Any tractor in decent shape that can't run at pto speed continuously needs to be repaired.