If we go completely logically, imagine you kill 400 marauders (some of the most favourite item-droppers). In doing so, you will notice that delivering hits at the joints hurts them more, slows them and makes them an easy pray. THIS is why your damage increases. However, killing even 4000 marauders might mean you will learn where they hide their valuable items, but imagine you kill a poor marauder who has nothing worth stealing. Then, even if you are the best pickpocketer, you can't find something that isn't hidden for the fact it never existed at the first place!
From the practical side. 8% more damage are nothing in tier 1 and 2 apparently for you. Now imagine fighting a mob in tier 2 (a regular mob of two or three opponents, the ones with five or six in tier 4 are not interesting). So, you have extra 8% against everyone, which is between extra 16% and 24% in total. So which would you prefer- finishing the battle about 75% times faster (thus sustaining 24% less dmg) or having extra 8% chance to find something.
1st case: extra luck: 4 battles, 32% extra
2nd case: extra dmg: 5 battles
And, since in tier 3 stages I find something in almost every battle, I think the chances of finding smth in the big zones are more than 33%... if you survive, that is. And these extra 8% dmg against any opponent might be the difference between death and victory sometimes.