This is really only a recent issue/trend.
In 2007/08, Chris Hodgson and Tyler Dietrich were the odd guys out. Chris was an impact player, but not a top 6 forward. Tyler came from UBC and played the one season. He wanted more ice-time and left for STU.
In 2008/09, Luke Lynes & Justin McCutcheon were the only two with didn't play the entire season. In both cases they were utility players with a scoring upside for Luke. He never played a complete season, some due to other players coming in but also because of injuries.
A lot of UNB players played less than 20 games in 2009/10, but most were the result of injuries and were mainstay players (not players they wanted to 'keep away' from others); Alex Aldred, Spencer Corcoran, Jonathan Harty, Josh Hepditch, Chris Hodgson, Luke Lynes, Taylor Procyshen & Ben Wright. Alex and Spencer were extra D after Ben came in at Christmas. Tyler was out with a concussion for half the season. Josh was a swing man for D and forward and Chris and Luke swapped spots on the 3rd/4th lines.
Only six(6) players had less than 20 games in 2010/11; Marc-A Desnoyers (came in at Xmas), Luke Gallant (injured), Matias Keranen (utility forward), Luke Lynes, Steve Pearce (utility DMan), Taylor Procyshen (concussion), Daine Todd (shoulder).
The only player over these four(4) seasons who you could argue could have been a top-6 forward somewhere else is Luke Lynes. He did go onto Waterloo and played 24 games with 6G+6A, but 12pts isn't a top-6 forward. He was 8th in team scoring and the 4th lowest Center.
In 2011/12 season is the one that proponents focus on. There are ten(10) players with less than 20 games; Jordan Clendenning (knee), Spencer Corcoran (7th defenceman), Chad Denny (Xmas start), Josh Kidd (concussion), Jeff Lee (injured), Luke Lynes (injured/utility), Stefan Salituro (utility), Ben Shutron (injured), Shayne Wiebe (Xmas), Ben Wright (injured), Geordie Wudrick (Xmas).
At the time, only Salituro could be considered a top-6 forward 'out of the line-up' as Shayne and Geordie played all the games after Christmas they could. The accepted rumor is Geordie didn't play and was lured to UNB to keep him away from other AUS teams - he played every game (but on the 3rd line). It did put Salituro & Lynes on the outside, but Lynes was injured for some of that time. Also, Salituro was a walk-on, he wasn't recruited.
In 2012/13, Wudrick leaves after Christmas, upset at with limited ice-time while Tim Priamo arrived to shore-up the D (Robertson, Kidd and Wright had all been injured at different times and Spencer Corcoran had not returned UNB this season). This results in a backlash that Wudrick could have been utilized by other teams in the league in a top-6 position. Possibly, but looking at his stats from UBC and that he was only there for half a season - not sure he would have stayed with any team for very long. As such, the argument that UNB horded him is a bit lame. With Wudrick gone, Salituro stepped in and played 18 games with 9pts.
2013/14 is the first year of the cap. UNB recruits Keevin Cutting, Shea Kewin, JP Labardo, Cameron Maclise, Philippe Halley, Philippe Maillet, Jordan Murray, Matt Petgrave, Mike Thomas & Ben Duffy. Halley shows up before Duffy and gets the last card. Duffy ends-up transferring to SMU over the Summer. Maclise wanted more ice-time (used on the 4th line) and leaves for MRU at the end of the season, Labardo upset with limited ice, leaves after Xmas for Ryerson but ends up in the ECHL. Cutting leaves over the summer due to personality conflicts with the coaching staff for Ryerson and Thomas leaves for Humber College (fire fighter). Without a cap, Duffy plays for Labardo and no one is in the stands.
Salituro could have been a 'diamond in the rough' and could have been utilized more, but he didn't seem to mined. He did graduate and went to Western where gets 16 & 29pts in his two seasons (his 29pts was 2nd best on the team and would have been 2nd on UNB as well - a team that won it's 5th national title).
So, to focus solely on Wudrick, Salituro and Lynes as the 'top-6' talent that was sitting and being horded by UNB is a bit of a stretch.
I have to say it certainly makes recruiting harder - a) do you go all-out in the Summer and not look to add at Xmas or b) get some now and hold onto a card or two for Xmas. In UNB's case, they certainly need another DMan, but what if a sniper shows interest? Do you ask Thomas or Braes to wait for next year.
In the end, regardless of your feelings for Gardiner - no other conference is doing it, so why handicap the AUS. I'd like to see the limit increased to 23.