I wasn't saying that they would play the same role. My major point was that I believe that they shouldn't dedicate so many roster spots to guys that can only contribute on special teams. Slater, Ebner and Hull feels like overkill to me.
It's a double edged sword. If you lean too heavily towards how the backup performs on special teams, then your defense (or offense) suffers when that player is called upon to take the field due to an injury.
On the other hand, if your roster is based almost exclusively on position play then your special teams are apt to give up a crucial, game-changing play. A good example of this philosophy backfiring was Bill Cowher's Pittsburgh Steelers from about a decade ago (think back to the Patriots-Steelers AFCCG leading up to SB36).
With 46 players available for the gameday roster, it stands to reason that a certain amount of backup linebackers, defensive backs, running backs and wide receivers be adept at special team duty - especially those that are unlikely to ever see the field of play as a position player.
Given the Pats' salary cap position, there wasn't much available for a veteran backup linebacker. Guys like Karlos Dansby and Donald Butler commanded big bucks as starters, and guys like Brandon Spikes were a bit high priced considering how one-dimensional they are. D'Qwell Jackson would have been a nice addition, but 4 years/22 million with $11 million guaranteed is again too much for someone that wouldn't start. That leaves players like Pat Angerer and Desmond Bishop who have huge red flags due to their coming off injuries - and because of those injuries would not be able to play special teams effectively, if at all.
Hull is hardly guaranteed a roster spot (it's either him or Chris White), and comes cheap (he surely signed for a vet minimum contract). If he makes the club he fills the same role others from Larry Izzo to Niko Koutouvides have done in the past: backup depth expected to play special teams and if called upon to play linebacker, hold their own while the starter heals up. It's either that or a raw late round or UDFA rookie (drafting a MLB early makes little sense) with no NFL experience at all; personally I'd rather have a guy like Hull or White.