The system works best if everybody rolls on every upgrade no matter how small it is. Even if the cost to benefit ratio of a trinket or weapon is that much greater, it will still end up in the hands of somebody else who will make great use of it. Class leaders and role designated channels should also aid players in selecting which upgrades benefit them the best and which don't.
Maybe the caster trinket isn't as much of a DPS improvement for a shadow priest as a mage, and Mininolan who was on top for the trinket took a minor belt upgrade soon before the trinket dropped, but Letholas is a great player and has perfect attendance.
If it comes down to several suboptimal upgrades or sidegrades for several players or one optimal upgrade for a single player, the cost of sharding suboptimal upgrades or sidegrades will never outweigh the single optimal upgrade for one player.
THEORYCRAFTQuote:So I used RAWR and shadowpriest.com to evaluate upgrades and use this week as an example.
Mininolan
Mini's new belt; 631
Mini's old belt; 525
Mini gained 106 DPS from the belt
REIGN OF THE UNLIVING; 638
Mini's current trinket; 407
Mini could've gained 231 DPS from the trinket instead
Letholas
Letho's old trinket: 419
REIGN OF THE UNLIVING: 578
Letho gained 159 DPS from the trinket
So Mini could've gained 72 more DPS from the trinket than Letholas, Mini also could've gotten more than twice his DKP's worth of an upgrade from the trinket as well. However in the end the raid gains 106 + 159 = 266 DPS rather than if the belt were just sharded and Mini would've gotten the trinket which would be 231 DPS. So thanks to Mininolan's selflessness our raid gains 35 DPS.
Of course there are a shit load of other variables such as attendance, like Letholas has 100% attendance and doesn't piss the raid off like Mini so more likely he'd be in raids with the trinket. The belt may have just gone to some other DPS meaning we'd get ~100 DPS upgrade from them and Mini would supply a 231 DPS increase from the trinket for 331 DPS total which would overall benefit the by an additional 75 DPS.