The following is what I've observed about how the Pats have used their PS over the years under BB. Other teams may or may not use their PS a bit differently, perhaps with more emphasis on potential development.
Most of the guys who are on the Pats PS seem to be there for scout team purposes only. That's their job and they don't really need to be able to develop beyond that (bonus if they do). IOW, they don't really need to be adequate positional players at the "varsity level", and they're most often not. They just need to be able to play a role mimicking players from the upcoming opponent, roles that often change from week-to-week (mostly for skill position players). They also need to be guys who the coaches get along with - and the active-roster players, too, since they use the same locker room. Some of those guys become relatively permanent PS "fixtures", returning to the PS for as many seasons as they're eligible to (e.g., Ventrone, Chris Barker).
This applies to most of the players on the Pats PS for most of the season. However, the use of some PS spots changes as the season progresses.
During cutdowns, one or two spots may be used to stash players who were destined for the active roster from the get-go - temporarily until other roster moves can be completed (e.g., Karras this season). Also, during the first week or two, there's often some churn as the Pats establish the base scout team that they want to work with for the bulk of the season, though they'll continue to pick up guys who become available as the season goes along in order to cover specific positional roles (e.g. Will Tye).
During the bulk of the season, from mid/late September through about now (early December), there's usually some churn involving up to half the PS spots, though typically about half the PS guys remain more or less permanent PS fixtures for the entire season (e.g., Cody Hollister, David Jones, Damarius Travis, Ferentz, Ryan Lewis, Riley McCarron). But that doesn't necessarily mean that any of these guys are being "developed" to eventually graduate to the varsity. For instance, it maybe says something about Jones and Travis that the Pats chose to poach "brand new" Nic Grigsby off the Ravens PS to take Ebner's ST spot (for at least three games) rather than promote one of them, even though they've been working with the team for seven months.
There are often new guys brought in for just a week or so. Sometimes it's an "extended tryout", but often it's to acquire a guy who can mimic a specific player from the next opponent or two (e.g., Kumerow). Once that job is done, they're waived and often replaced with a guy who was on the PS earlier, if he's still available (e.g., Wiltz, Purcell, Jason King), to resume his regular scout team role.
There's sometimes a guy or two who will be promoted to the active roster as a temporary special teams fill-in for a game or two, then waived back to the PS (e.g., Ventrone, Grissom). Again, this doesn't necessaily mean that the guy is a potential permanent graduate.
Sometimes they'll use a spot as a temporary way station for a guy who they specifically bring in for the 53-man in order to get him under contract until they can make the transaction that opens an active roster spot for him (e.g., Reilly, Reedy).
Beginning in December, the Pats will often sign a guy or two to the PS who they may be interested in having on the 90-man roster at the start of the following season under a futures contract, partly to assure a base of positional depth on which to begin building that season's final roster. Hardison may be one of those guys. Ryan Lewis may have been an early-season pickup in that regard, since they're paying him so much extra. That may have also applied to Blackson and Foster (before they were poached), though either of those guys might also have been considered dire-emergency backups.
Anyway, a couple years back, I did a deep dive into the Pats PS roster since BB became HC to try to get a total of the number of players who'd been on the PS at one point or another over the years. I quit when I got to 200. Of those guys, only about 10 have ever graduated to permanent active roster status (a full season or more). So, the all-too-common notion that the majority of Pats PS players are "varsity in waiting" really doesn't hold up to scrutiny.
