Not so quick answer:
1 - It is expensive, i.e. one pound of Laterite is about the same price as 50lbs of Soilmaster or Turface, or one package Flourite, all of which contain iron.
2 - Take a look at this link: http://home.infinet.net/teban/jamie.htm
You'll notice that both Flourite and Turface (Soilmaster is a similar product as Turface) have a decent iron content when compared to Substrate Gold (laterite). First Layer Pure Laterite does have considerably more iron than Flourite or Turface but how much is enough?
3 - The Soilmaster may not be "heavy" enough to "cap" the laterite. I didn't want to take the chance of the Soilmaster not being an adequate "cap" for the Laterite.
4 - I already dose the water column with a micro fert that contains iron so why add more?
By dosing the water column I don't feel there is a need for the addition of any macros or micros to the substrate. I don't use any substrate fertilizers in my tanks and don't see the need. I feel that plants will grow just a well by fertilizing the water column. For those who are neither willing nor able to dose the water column consistently, I would recommend the addition of laterite or better yet, Eco Complete.
I don't feel any of the "commercial" substrates curently available are adequate in a high light ( more than 2 or 2.5wpg) tank. The plants seem
to grow too fast to rely on their roots to provide nourishment. You can easily test this yourself by watching the growth of a tank with high light and Eco Complete substrate by skipping a few fertilizer additions to the water column. After skipping 2-3 doses, (maybe more in a lower light tank) the plants will begin to look worse (smaller leaves and or stunting) and algae will begin to grow.
By all means feel free to give laterite a try. Just make sure the "cap" you use is heavy enough to not allow the laterite to seep up through the substrate. It can be a real mess if the laterite leaches into the water column. That heavy orange tint (like green water only orange) will take all of the enjoyment out of watching your tank and it can't be filtered out with a diatom filter like gren water can, at least until all the laterite has leached into the water column