Yes it works better for a QB as he less likely to suffer a serious injury, but it can work for anyone. Bell could have signed the second tag collected the $15 million, increased his market value. Surely would of been a better move then what he did. If a injury happens he has the $15 million and can rehab and be a free agent asking for big bucks again and likely get it. You see Thomas wanting be the highest paid safety after his injury last year? Like that, only Bell is still young. Fearing the franchise tag is a mistake. If the NFLPA wants to address it the NFL will leverage that to their favor, because it's stupid for a NFL team to use it twice in almost all cases.Cousins situation worked because of the position he plays.
In Bell situation, he plays RB where teams consider you done soon as you hit 30. The top QB's are usually playing at a high level well into their mid 30's
There are better causes to fight, for the players.