It comes down to how good your team is. If you are 5-2 or better, stacking injured players makes a ton of sense, although I hate SJax, guy is like cancer to anyones FF team. If you need to win now and your team sucks, you need to take a few fliers and hope for the best... A guy like SJax may have some decent trade value if someone wants to buy the name, so if you are sitting at 4-3 or worse, may make sense to shop him.
Harvin may go this week, though I personally doubt it - When he comes back say week 9 or 10 he will be a difference maker, I've got him stashed too.