On the PBE, Late game ADC's have 50% or more lifesteal, 300+ AD, and 70-100% crit.
Lifesteal and physical damage vamp is frekin' everywhere. 2% in the masteries, another 15% on crits in the masteries, 15% phys vamp on red pot, 12% lifesteal on Mercurial, 12% phys vamp on a new top - tier AD item, and 10% on Maw.
With ye standard 5 items - of IE, BT, SS, PD, Red Pot, and Mortal Reminder (Last whisper + 5 seconds of grievous wounds on all physical damage) you are at 210 bonus AD, 80% crit, and 52% Lifesteal on crits, 37 off crits. You still got a slot for more CRIT, AS, and MS, or Mercurial or Maw or whatever suits your fancy.
Most Casters are going to go IE, ER, SS, upgraded LW, BT, TF, Red Pot- 90% crit, 40% CDR, 50% Lifesteal, 300 bonus AD, 10% MS (slightly slower than boots, slightly faster with phage proc), while corki will drop the upgraded LW for whatever he damn pleases.
Seriously, ADC's get 30% to 32% lifesteal late game WITHOUT A SINGLE LIFESTEAL ITEM. Admittedly, 15% of this (the red pot) only works against champions.
At least their ADC applies grievous wounds too, although it's down to 40% reduction.
(To all the people who think I'm saying that ADC's are in a good spot right now (they aren't), or that I don't want marksmen to be fun to play, please read my post here!)
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and several other bursty champs.