Why should the scientific community avoid double submissions? Every journal has strict rules about duplicate submissions, and I do not mean publishing the paper in 2 venues.

My approach is mostly from an economics point of view: Given the fact that most reviews take between 2-3 months, plus any number of months for a resubmission, limiting yourself to only one journal seems not only a waste of time for the individual, but an overall drag for the scientific community. Even if you make a groundbreaking discovery, it won't get published until about a year later.

Multiple submissions dramatically increase the chances of getting your work published, and if you get accepted in both, you can always pull out your paper. From a marketplace point of view, this makes sense, since in this way, journals would be fighting for authors and not the other way around. 

Models like Arxiv have proven that this is not such a crazy idea
One possible answer is that the referee process of a paper is a very professional and time consuming job (at least I am sure it is in mathematics). Therefore it is not fair you submit your paper to several journals and make them to referee your paper by different experts and then you withdraw your paper just because your paper got accepted by another journal.