Healers are another concern, so if you have a group of 15, and only 3 healers, would you bring another DPS along without a 4th healer? The thing is, these latter issues are far harder for Blizzard to fix, and they are inherent problems of the Flexible system. There's no blanket resolution for these problems, only individual mechanics to tweak and alter.
But is this really a problem? Ion Hazzikostas is quoted in various places as saying that the fights are tuned very sympathetically so that the answer is never to bring fewer people, but it's still been the case in groups I've been in that dropping under-performing players has been the difference between scoring a kill and not scoring a kill. Should the tuning be even more sympathetic? It seems pretty straightforward, as long as most of your DPS are outdoing the tanks on single-target, but wipes are not something the community wants to tolerate any more.
There are complaints in the aforementioned thread that players are demanding a 530+ ilvl for PuG Flex, or demanding normal-mode clears, or other qualifications for entry. This is always something that is going to happen, along with bringing numbers that some theory or other deems "best". Players are always going to min-max, they're always going to try to take lower difficulties more seriously than they're intended to be, particularly if they award decent gear. As we discussed recently here on WoW Insider, we're getting more wipe-averse, and that makes players go to ever more extreme lengths not to have a wiping PuG.
This isn't what Flex is for. Flex was never really designed to be PuGged, any more than normal modes are. Sure, it's possible, but taking Flex so seriously that you're demanding high ilvls and hardcore players, while min-maxing on mechanics isn't using it as Blizzard intended. Saying that, though, the original design intent of LFR was to specifically avoid using as it as a rung on the gearing ladder. That worked out well, didn't it? So remember, when you're building your groups, that Flex is designed to be fun for "Friends and Family" guilds. It's there to be inclusive, and entertaining. Not to be min-maxed. And then forget all that and do it anyway... we're gamers after all.