When I make lunch out of breakfast stuff, it's generally cold sausage butties on toast or bread (depending on how good the bread is, I like cold buttered toast!)...
I usually have a sachet of ketchup for it too.
It's never beyond the range of what some folk could eat in one sitting though, I can't eat a huge amount at breakfast.
I only do this if im attending a conference at the hotel/at somewhere really nearby though and it goes into napkins/sandwichbag - I've always got my stuff ready for the day before going down to breakfast so I have baggies and a laptop case that keeps things flat anyway.
If i were at a hotel to go out and about and do stuff, where its easy to buy lunch, I wouldn't bother, but conference lunches or training course lunches tend to be pretty boring, in my industry, often vegan or spicy and therefore often has options I don't want and no alternatives to buy from anywhere/nowhere to buy them.
I see plenty of other people doing it, rarely do I see anyone taking the micky - but i DO see people piling plates high with stuff they can't eat and then just leaving it, thats really common.