If you ranked WR at the moment you have two Veterans (Moss, Welker) on top, followed by three youngsters (Edelman, Tate, & Price) on a probable depth chart. Tate and Price have both flashed talent in preseason, but not consistency. You may argue Aiken's overall playing level as a WR, but he plays his position well enough for a Special Teams' Ace to balance out the inexperience the other three reserves bring to the field. Thus before factoring in needs for Special Teams and for other positions, Aiken's value as a veteran WR brings him into play for a roster spot.
Now factor in Special Teams. Aiken plays on all the units. He can and does play multiple positions on those units.
Chung can play the same roles as Aiken, but Chung is also pegged to be a starting Safety, he's redundancy for Aiken's roles, but his primary role will impact how much time he can put into Special Teams.
Arrington plays on the coverage units and as a Vice on the PR unit. He is a valuable ST's player, but he doesn't completely replace Aiken.