Lets take LFR:
30 players, 6 of each armour type (30 players ÷ 5 armour types), both PL and GL drop 1 piece per 5 players so 6 drops total, lets say 13 total boss drop (cloack, ring, neck, 2 of each armour type)
Lets say you want plate shoulders
Personal Loot
Chance to be choosen for loot:
1/30, 1/29, 1/28, 1/27, 1/26, 1/25.
This comes to roughly 21.9% chance at loot.
Since your loot pool would be 5 items (ring, neck, cloack, armour piece 1, plate shoulders)
21.9×(1/5)= 4.38% chance to win plate shoulders
Group Loot
Probability of drops: 1-chance of it not dropping to the power of amount of drops
1-(12/13)^6 or 38.1% chance at seeing the shoulders drop.
Since youre rolling agaisnt 6 plate users
38.1%×1/6= 6.35% to win plate shoulders.
Actual Point
From the quick math and the rough example, Group loot has a higher chance at winning a piece over personal loot.
If you look at how the game decides what drops, rings, cloacks, and neck pieces make up 60% of the loot pool for each drop in personal Loot compared to the 23% chance to drop under Group Loot.
Yes daggers may drop under Group loot for no rogue, but that is a side effect of having a more diverse loot pool. With neck, cloaks and rings being able to be dropped for everyone, they make up a larger amount of dropped items under personal loot.
TLDR:
Both options have problems and niether is perfect, PL with stacked raids and GL with visable rolls (emotions and feelings) and many other issues that would take more time to write. Hope this helped a little so you can understand where others are coming from. I can write more when im off work.