For the last several years and the foreseeable future, lifelink has been and will continue to be a static ability that changes the result of damage such that damage from a source with lifelink also causes its controller to gain that much life. Long ago (before M10 rules), lifelink was a triggered ability, and having multiples meant something. The one that seems to be most commonly confused is lifelink. Things like Trample, First Strike ( Striking Sliver), Double Strike ( Bonescythe Sliver), Infect, Flying ( Galerider Sliver), Reach, Deathtouch and Haste ( Blur Sliver) don’t perform any differently if a creature has one or twenty copies of the ability.
However, there are also many abilities that are useless when duplicated. Static abilities that modify power/toughness (like Predatory Sliver or Megantic Sliver) also make cumulative sense. Generally speaking, it’s the triggered abilities (like Exalted or Flanking) that make sense to have more than one. There are some keyword abilities that are useful in multiples.
