In teams, lots of charachters have tricks up their sleeves that can make them highly valuable. For instance Fox/Ness. I know, you're thinking "Ness Sux, Lol!" However, with fox recovering ness's health, ness can be an incredible stock-tank. His throws generally hold an opponent out where they can be whacked, and since teams is a lot more chaotic than 1v1, a ness has a lot more survivability. If anyone was at INNsomnia 3, there were ness's on each team in the teams finals.
Of course, success depends on the teams opponents. I once played the Luigi on a Luigi/Marth team, and we did fairly well against a Peach/Falco. (A close loss). Then, in the losers bracket, we got absolutely ***** by a shiek/shiek. Low tier charachters seem to be very limited by matchups in teams, but they've got more opportunities to shine than in 1v1.
I'm not saying that you should go form a Young Link/Yoshi team, and expect to win MLG Doubles, because you'll lose. I'm saying that as counterpicks, combined with stage selections, low-tier charachters can be a viable choice.