No crew has to ask your permission to add fuel to the flight. ... Sure, that would be the fuel "just cuz". I have never met a pilot out there that adds fuel for no reason... However, I think your mind is a little warped on this subject. Remember that you aren't the one in the aircraft.
Depending on the rules of the airline, the captain might have to request dispatch clearance for any fuel added above release minimum. Some carriers give the captain a certain amount of minimum as a captains fuel, above which he has to get dispatch clearance.
I have seen captains add fuel for no reason. A friend of mine, dispatcher at one of the big-3 carriers, had this captain that out of habit added a thousand #, so the DX added the captains thousand, and then when the captain signed the release, he added an additional thousand - the captains Just Cuz fuel. The flight now had 2000 lbs of unneeded fuel (SMF on a VFR day) ontop of reserve and the standard contingency - a B737 was landing at SMF with 7.0 on board for no reason.
Hey, I am all for adding gas if it is required. I wont dispatch a jet on a transcon into ORD with only 1000 lbs of contingency (except maybe in the middle of the night); I know how ORD ATC can get screwy from those seemingly inconsequential ceilings and flights can end up with a 50 mile downwind. But landing with 90 minutes fuel on board for an Indianapolis, a Dayton, a Sacramento, in severe VFR; that IS wasteful.
My mind is a little warped only from DX management chewing my ass for adding too much fuel when it isnt required. You havta keep in mind - we're always within their sights. If it is operationally required, sure, lets figure out how much and add it.
As a dispatcher, I dont like to divert anymore than you do - it screws up crew times, crew rotations, aircraft routings, everything. HOWEVER, I do consider the occasional diversion to be an acceptable cost instead of carrying 45 minutes of fuel ON TOP of reserve (and alternate if required) for no justifiable reason; especially when Jet fuel is 4 bucks a friggin gallon.
Oh - and Hamburger, I never wanted to be a 121 (or 135) pilot, I prefer to sleep in my own bed (or crashpad bunk) nightly. And Ruskie, I have refused aircraft on occasion when I just dont like what the combination of weather, and maintenance is handing me.