All attack speed bonuses from leveling up are added to item bonuses, not multiplied off of by them. So you start with 100% attack speed at level 1 and end with around ~150% attack speed at level 18 - as such, the attack speed bonuses from Phantom Dancer end up being only about 66.67% of what they display as the game progresses. Also, attack speed is sort of the retarded cousin of all the AD attributes: damage, critical strike chance, and armor penetration will all see a solid bonus to your damage when you're harassing or kiting enemies, were you generally have to sit still for attack speed to count. As such, it's generally not advised to rush attack speed, unless you have really powerful on-hit effects that merrit doing so.
Also, Tristana has an attack speed steroid - which also rides off her base attack speed, and doesn't multiply item attack speed bonuses. Attack speed has no synergy with attack speed steroids in that sense - but does work rather well with things like attack damage and critical strike chance.
Beyond that the reason you'd rush items is for the goodies from completing them. Phantom Dancer's unit collision is nice, so is the movement speed - but neither are that core on an ADC, at least not right away.
It's great to experiment with builds, and probably doesn't matter much what order you get the items until you're Elo gets to the upper levels - but the above is mostly the reason you'd get Infinity Edge first, and why rushing Phantom Dancer generally isn't the best idea - unless you're Vayne (silver bolts and the final hour AD steroid) or Kog'Maw (extra range and that health and armor shred on-hit).