Honestly, my inclination was to leave the as-is for the random roll and standard array options - partly because I would find myself torn between 17, 15, 13, 12, 10, 8 or 16, 16, 13, 12, 10, 8 (assuming a standard +2/+1 race). Of course, you could offer both.I agree. Floating scores aren't a good idea IMO, but the problem is rolling those into point-buy works, and you can always boost the standard array, but how do you incorporate them into the rolling method? I've looked into it, and I think rolling 7 or even 8 scores, and then using the best 6, does this effectively.
As you say, you could build it in to random rolling by having players roll extra stats and then choosing 6, or perhaps by having them use 5d6-keep-3 for a couple of the stats. Either way, this does have the advantage that the maximum for a starting stat is capped at 18, which the current "roll then add bonuses" doesn't give you.