A dimmer in the roof space? Seriously?
I saw no reason not to.
When I switch on a heater, l like to adjust the the temperature of the room to what I like.
When I switch on my TV, I adjust the sound to my preferred level, depending on my mood I guess.
When I switch on a light in my home, I prefer to have the light level I want and not being limited to the wattage level of the bulb used.
All of the lights in my home are on dimmers, except for the entrance hallway, vestibule exterior light and downstairs toilet (but that's mainly because they are on a three switch 'light switch' and I don't feel confident to upgrade to a triple dimmer switch).
I like the 'control' with dimmers and would have it no other way.
Given that this appears to be a trussed roof, why do you want a light up there?
well, there's at least some plumbing up there, which could need maintenance - not so easy in the dark or when holding a torch in one hand.
JohnW2 basically answered this for me; there is satellite TV cable, normal TV cable, plumbing and electrical cables in the loft. I have had to go up a few times to do jobs and have found it a most unpleasant place to be in.
In addition, not being able to see where you are going (unless you give up one of your hands for a torch) I find to be potentially dangerous, particularity when putting a foot wrong can send you down through the ceiling.
Having both hands free; being able to maneuver easily and being able to see clearly are all added benefits to performing work in the loft area - me thinks.
It makes me wonder why house-builders don't put loft lights in as standard. I doubt if they would get any complaints from their buying customers.