The problem is they're additive, which is essentially useless at 10%. A ghost world has enough of a bonus to make it a high incentive to specialize the planet since it's basically a free tile, but with the production buildings being percent modifiers themselves, the bonuses would be better if they were on the gross instead of the base. A fully ringed +1 adjacency bonus tile is already better than even that ghost world, giving 60%. If you have two +3's, or tiles you can ring, the ghost bonus is almost useless once you specialize in another facet.
As it stands now, food, raw production, and even approval bonuses are massively more powerful than research, economy, and manufacturing. A 50% food bonus planet lets you obtain massive populations, potentially creating hundreds of extra productivity on good planets, while even a 50% ghost world is only going to net you around 60 points even on 20+ quality planets.
