League of Legends XL: Season on Hold 'til Morello gets Gold

[ex cathedra]

They're all purposefully gold-inefficient items that provide stats that don't enhance each other in any way. Most mid- and top-tier items are very efficient in that they give you more stats than you paid for, and many low-tier items aren't particularly efficient at all, Doran's excluded. This creates game states wherein the first champion to finish a "big" offensive item (because that rule of thumb doesn't hold true for defensive items; stats-wise, negatron and chain vest are ridiculous and most of their upgrades mostly provide utility rather than raw and efficient defenses) has more effective gold than other champions. This is one of the several gameplay mechanics responsible for how easily League tends to snowball. S3 changes are, in part, aimed at reducing this.

GP10 aren't "real" items, especially after the Philo/HoG nerfs. Avarice Blade (on average) adds less damage to your auto-attacks than even a simple Doran's Blade up until you get more than 120ish AD. Even on Q, where it actually shines, the bonus damage boost is inconsistent and worth about as much as 10 AD at rank 5.

If you come to lane with Philo or Avarice and your enemy comes to lane with 2x Doran's blade or 1x Doran's Blade + Ninja Tabi they can all-in you and you can't do anything about it. If you are playing against enemies too incompetent to realize this, you're settling yourself into a poor playstyle that won't work nearly as well against the better players that you'll inevitably face.

You're over-paying for stats that don't synergize with one another while your enemies are under-paying for stats that multiply each other. It's just math. 4x GP10 is weak because GP's early game is weak and he can't carry those items with just his base values. His Q costs too much mana and Ninja Tabi reduce too much damage for you to spend any amount of time harassing with zero-item Parrrley's once you've had the chance to spend some gold.