I'm doing a Joint Honours Batchelor's in Chemistry & Management.
The major Con is that I don't study Chemistry in sufficient depth to be recognised as a scientist by the Royal Society of Chemistry - hence there's no chance of me ever getting a job in research.
The Pro is that by studying management, I get a broader horizon, developing skills beyond the mere analysing that science teaches you. Also, given that for many graduate jobs in the UK you don't need a specific degree (i.e. non-science, non-engineering or non-hard-core technical roles), the extra breadth of experience is an advantage.
Personally, I think it's excellent that I've mixed science with a non-science field - I see both worlds.
I don't see why double degrees have to have any less depth in their course than a single degree.
In States.. You don't have to take the courses which can be counted as meeting requirements from another degree.. i.e. most of my electived were either environment related or business related. If I had taken a degree in business.. I would not have to take a lot of courses as I had already taken them.. but as it is a different discipline to take all the necessary courses to graduate with that degree it will take two additional years.. compared to one additional year for degree which is related to my field - engineering.
I.e. it would be quicker for me to do mechanical and industrial as a lot of core requirements are same.. same with computer science and engineering.. many of the core courses are same. and it takes only one additional year. one of my friends did mechanical and industrial while other did comp sci and engineering.