The 6'2, 235 lbs Singleton spent the offseason with the Seattle Seahawks as one of their rangy outside linebackers. He has an explosive 10' broad jump and a great 4.65 40 yard dash. He comes out of a small school in Montana State and will serve as depth on the practice squad. Singleton is a quality run defender but is very raw in pass coverage. He is also slightly smaller than most Bill Belichick-style linebackers.
It is telling that linebacker James Morris is still a free agent after the Patriots waived him.
Barker was waived after the Patriots brought back offensive tackle Cameron Fleming to the practice squad. The Patriots also have tackle Blaine Clausell on the practice squad, in addition to depth players like David Andrews and Josh Kline on the active roster, so New England has plenty of scout team bodies. Linebacker was clearly a bigger need.