I think it's a nice idea in principle to help repay the loyalty of long-serving players but adds another layer of complications to an already complicated Salary Cap. I also think it's something that is open to abuse.
Taking your example of 8 years service and/or an academy graduate who has stuck with the club you could exempt half of the Sarries squad, allowing them to spend even more on incoming talent.
I think for it to work it needs to be age-based rather than service-based. Perhaps the same criteria - salary exempt but can only play 50% of league games (and a 1 minute appearance in a single game counts as a game) for those who are 33 and over.
That way the players exempted aren't in their prime years of their career (in theory - a few exceptions like Gopps, Stringer, Nick Evans etc...).
But for me, it still adds too much complication to an already complicated cap structure.