Given the subject line, a link to the relevant article was anticipated within!
As with many other variables in training, the short answer is: it depends.
It has to do with both goals and with the realities dictated by one's choices in life, and even possibly health issues. Whatever the 'research' may indicate, for getting into great shape ASAP doubles are the way to go. Coach Mark Wetmore has his CU runners, as a rule, run singles because he wants to allow them the time and energy to be dedicated to their studies and to campus life as well-rounded individuals while still getting the most that they can out of their training sessions. I know that Pfitzinger and Liquori & Parker, Jr. do not recommend doubles for volumes below 70 miles/week (more or less), yet I also know that their advice does not reflect the way they trained and that the core market of buyers for their books is not made of people who would likely be as into running (at least initially) as they were and want to train the way they trained. It is also a blanket statement that fails to take into account the variables referenced above.
Suppose runner x is plugging away at 40-50 miles/week all year and cannot figure out why after huge gains in his first few marathons he is having trouble whittling away at the last 20 minutes between him and a BQ. Also suppose that he is doing that 40-50 miles/week in the hour he can fit in before work each weekday and in the three-hour window he can fit in on Sundays. Additionally suppose that his employer allows him an hour lunch each day yet would let it stretch to 90 minutes if he either came in 30 minutes early or stayed 30 minutes later in the evening or came in for 4 hours on Saturdays. On top of all that suppose that his theoretical s.o. goes to cardiyogalatespin at the Y 2-3 times a week in the evenings and could even take any theoretical dependants along and get them some playtime at the Y while she is there. So runner x, despite being ingrained in running five times a week within an hour window and then once a week within a three-hour window, could have a lot more flexibility to add volume (and spread that volume out more evenly) to his training if only he inquired with his support network and took advantage of opportunities that might be available. He could be keeping the morning runs 2-3 (if not 5) days of the week, picking up five midday runs, and possibly adding 2-3 evening runs in addition to pulling the longest run back within reason – either as a lower duration or as a lower percentage amid the added volume on the other six days of the week – and adding a run on the other weekend day to get to a training volume that would really let him approach his physical potential. Even if he just added 30'-45' daily midday runs to the hour morning run routine he would be doing himself a huge favor.
Often it is not simply a matter of extending the runs within the existing training regimen to get up to whatever maximal amount one could do in singles, it is more a matter of creatively using the available free time within one's usual daily routine. Many are simply not in a position to “maximize the training benefit” in the most ideal manner so it is more about choosing among different compromises. At the level of the typical running magazine reader and running forum commentator, it is something of an exotic topic. It would be akin to reading about how to get the best performance out of Ferraris and Vipers when one is a lifelong Accord driver. Not that one should not aspire to drive a Ferrari, just that there are more essential basics to master on the way to good driving performance within the constraints present in an Accord before worrying about nuances of nimble handling and fuel injection timing. Sort of like the “central governor theory.” That would be something to be concerned with once one is absolutely certain that one has already plateaued in performance through a mix of ideal volume, ideal workouts, and ideal recovery. For many, being concerned about maxing out what could ideally be accomplished via singles before incorporating doubles overlooks the fact that running more is running more and is the basic avenue to improved fitness. If sleep patterns and time slots to which one has obligated oneself will not allow large enough blocks of time to extend what can be accomplished with singles (within a smartly proportioned training routine) then by all means do not let that stop one from seeking out additional sufficiently large blocks and extend the overall volume with additional runs.