I think this is more about preventing dead leg than anything. Fields could be hurt, but this would be my guess if he isn't. http://www.nationalfootballpost.com/Wilson-Dolphins-adding-Britton-Colquitt-to-practice-squad.html
Perhaps he is just an insurance person until the season ends. Then he would provide competiton for Fields before they cut Colquitt.
Dolphins released guard Mark Lewis The Miami Dolphins officially cleared a spot for punter Britton Colquitt today, releasing rookie guard Mark Lewis from the practice squad. An undrafted free agent out of Oregon in 2009, Lewis spent the entire offseason and preseason with the Dolphins, but was waived prior to the regular season. Lewis also spent a week on the practice squad of the Seattle Seahawks in October, but was otherwise a free agent during the first two and a half months of the 2009 regular season. Lewis chopping down Christmas trees for work in late November when the Dolphins came calling with an open practice squad spot. It's no surprise to see Lewis get the axe (pun intended), as there were two guards on the practice squad, Justin Smiley is healthier now, and Lewis is barely a pro prospect to begin with. With the recent moves, here is how the Dolphins' current practice squad looks (rookies in italics): CB Will Billingsley (injured) P Britton Colquitt RB Tristan Davis G Ray Feinga LB J. D. Folsom WR Taurus Johnson TE John Nalbone FS Nate Ness WR Julius Pruitt Most players that finish the year on the practice squad, if not all of them, should be re-signed to the offseason roster in 2010.
