I am not sure I understood what you meant here, but point buy in itself does not give me any chance (it is exempt of chance), whereas rolling does give me a chance (as chance is involved after all) to have better ability scores than point buy does. Just by giving me a chance, any chance, to roll higher, it already gives better chances, does it not? Of course, it also gives me a chance to have worse ability scores than point buy. I am not stating I have better chances to play a concept I would be playing no matter what my stats are, what I am saying (and what I guess [MENTION=6789021]Yardiff[/MENTION] meant) is that I have a chance to try a concept I wouldn't otherwise, or that would be too gimped, as it would not really work if I either rolled poorly, or average, or did point buy/standard array.
I can understand that, if I am wholeheartedly invested in a concept, then rolling might help, or totally get in the way of my fun, and the netting an average so close to point buy just means point buy will actually more often fulfill a minimum required for any generic build to be feasible. But from a perspective of someone who will play dozens of PCs over the years, and does not feel invested in any one single concept for one specific campaign, but has many concepts in mind and can use any of then, rolling just feels like extending the horizons.
I am trying to say is that what I see in play is that rolling allows for concepts out of the obvious. Sometimes because one needs to work around bad stats, they end up building some PC that does not really depends so heavily in stats to begin with. Sometimes because one can go for the underdog concept that would not really be effective in play given what point buy/standard array allows, but a higher than average roll makes it work.