I'm one of those on the leaderboard (I've been as high as #2, now I think I'm #5), though that's really not why I've been contributing.
I'm on odd hours, and often when I am there are holes in the short duration missions. I've probably spent 50M credits buying up green diplomats and white chefs, etc. just so we can get the assignments going and not sitting there waiting. The quicker we get this done, the more quickly the starbase gets built and we have all the features everyone is waiting for.
Personally, requisitions are a really minor thing for me - I have all the C-store ships, eventually I'd like to pick up a Fleet Science Retrofit, but we are a long way from Tier V and being able to buy those.
That said, it would be nice to be able to buy something with the fleet credits I've accumulated. I saw some provisions for armor/weps on the vendors - but can't yet buy them because I'm not high enough rank. I'm not greedy - LOL. I just want one piece of armor and maybe a new gun. Hopefully something can be worked out so people who have contributed and have Fleet Credits to spend can buy limited items.
As to people missing out on contributing - well, you do have to plan a bit. Pay attention to what is being asked for with specific missions so you've got it on hand when the missions come up. The 3 main ones take 20 hours to complete - so yeah, you kind of do have to plan for them. And the 30-min missions, set a timer when you log on for the next one. Have something ready, there you go.
I'll be honest, more often than not there are only one or two people contributing stuff at a time, so it's physically impossible for anyone to "ninja" it all away. But if no one else is going for it, of course people are going to fill it up if they can - to get the mission going, so the next one comes up, and we can get the Starbase on it's way to completion. But, like anything in an MMO, you need to plan ahead a little, and be flexible when you can log in if you want to be at the forefront of new content.