So in summary: power, synergy, flexibility, and reliability
The mana gain is awesome, only in a five-color deck, you will likely have to use that extra mana as colorless mana and will still need mana in other colors
The cards have a "1999-2000" copyright and slightly glossier stock, though importantly, the holos in those packs reused 1999-dated foils, so a 1999-2000 copyright on a Fossil non-holo is the only way to identify the third print directly
Absorb would not be likely to revolutionize the Modern format or anything, but it would provide another fringe option, like Counterflux, for tweaking ones deck for a particular metagame
If that creature is a legal target but the creature you don't control isn't, the creature you control still gets +2/+2
Modified Not everyone is lucky enough to be bitten by a radioactive spider