I know that a lot of people believe handcuffing is a mistake, a waste of a draft pick and roster space that could go to a player who isn't guaranteed zeroes while the handcuffed player is healthy. I know that others swear by it, and if they have three starting RBs on their squad they match them up with three handcuffs, no matter the circumstance.
The right things to do lies somewhere in between. We understand that a handcuff is insurance, so let's think through it just like we would insurance.
Whether I insure something depends on a number of things.
1) The value of the item and effect of its possible loss to me. I'm going to insure my brand new Escalade with full coverage if I'm the typical guy, because it's a big investment for me. I'm not going to insure a $1,000 beat up '85 Buick for anything more than the law requires, because if it gets totaled I'm not out much. Why bother? Also, I may not bother if I'm Bill Gates, even if it's an Escalade. He could push 10 of them off a cliff every day and not be impacted financially.
2) Cost of the insurance. In the real world, insurance has to come at a reasonable price or no one would buy it. There are competitive and regulatory forces at work, etc. But all that aside and in a fantasy world, if you had an item you wanted to ship across country that was worth $10,000 you'd want to insure it... but not if you found out the cost of insuring it was $7,000 (absurd, I know, but it's for illustration purposes only). There is some cost at which you insure, and some cost at which you decide you'd rather just take your chances.
3) Trust in the insurer. I may have a valuable item worth insuring, but what if I can't rely on the insurer paying off? What if I find that fly-by-night insurers turns out to be just about impossible to collect from or doesn't deliver on its promises? The insurance I bought turns out to be pretty limited. Or, what if they go out of business or are shut down for being fraudulent? My insurance becomes worthless.
I need to be sure I have an item worth insuring that will be painful to lose, insurable at a reasonable cost given the worth of the item to me, and have to be able to trust that the insurer will come through when needed. If I can have those things, I don't mind spending some money on the insurance rather than on something else in my life. In fact, if those factors are there, it's foolhardy and risky to ignore the value of insurance. It's irresponsible not to buy life insurance, for example, if I'm a young family man, the sole-breadwinner, with lots of debt.
But if I don't don't those things by buying the insurance - item worth insuring/painful to lose, reasonable cost given the worth of the item to me, and trust in the insurer - I'll just go do something else with my money and forget about it.
So, back to fantasy football and handcuffing. If I'm a dynasty owner of Tomlinson and losing him to injury will pretty much kill my season, I'm a fool not to take reasonable steps to get Turner. Do I get him in the 3rd round? Of course not. If someone else is going to grab him in the 3rd, they can have him. I'll take my chances. But I'm not going to be out there screwing around getting my PKs and WR5s when I should have secured my insurance. That's like the young guy in my example above spending his money at McDonald's instead of taking care of his family.
On the other hand (another exreme example), if I already own LJ and LT and SAlex and Portis, I'm not going to lose any sleep if someone else gets Turner. Yeah, I'd hate to lose LT and it'd be nice to have Turner, but I'm OK if someone else gets him. I have other needs.
Or, if I have Ahman Green (my '85 Buick) and I'm not sure if I might be dealing with fly-by-night insurance in Gado and Davenport, not even knowing who it will be or if they will deliver, I'm not going to be willing to spend much capital on them.
As to the players asked about, the whole shape of the roster has to be evaluated before the question can really be answered, but...
L Johnson - (1) LJ is a very valuable commodity and except in rare cases (mature dynasty team that's loaded at RB) it will be very painful to lose him, (2) Bennett can generally be had at a reasonable cost given the value of the player being insured (ADP around 190, RB60), and (3) though never an elite RB (or he wouldn't be where he is), he's got enough ability and a solid enough offense around him to be trusted to put up decent numbers if called upon. I'd generally handcuff Bennett to LJ.
T Barber - (1) Tiki is a notch below LJ and not as costly, and since he's drafted a few spots lower it means the 2nd RB drafted by the Tiki owner is possibly picked higher than the one drafted by the LJ owner. Still a valuable commodity though, and definitely worth insuring if the cost is OK. (2) Jacobs is around ADP 160, RB 48 so his cost is kind of high for a handcuff but he has value aside from a Tiki injury because he's the goal line guy. So I think that's a reasonable cost. (3) No one knows if he could do the job if pressed into service full time. He could be Jerome Bettis or Ron Dayne, we don't know. The Giants seem to believe he could and (I think) are looking to him to take over once Tiki retires to ESPN studios, but he's never done it. You have to decide for yourself on that one, but I guess I'll trust what the Giants are seeing in him.
So, I vote yes on both guys as far as handcuffing goes. I don't think that Bennett in the 15-16th or Jacobs in the 13-14th is foolish, given what you're insuring and what you'd otherwise be spending your capital on at that point. And they both should be able to do the job, though Bennett has shown his upside to be limited and Jacobs has both upside and downside unknown. I'd handcuff Jacobs to Tiki before Bennett to LJ, though.
And McGahee's handcuff? Go to McDonald's instead.
CP
