I think that the changes in prestiging starting with Black Ops have been influenced by Activision. If you think about what it takes to make the game compared to what it takes to make the map packs it's clear that profit margins on the map packs are huge compared to the game itself. So how can they get more people to buy the map packs? Encourage them to play the game further into the year.
Call of Duty 4 was the first time they implemented the prestige system, and World at War basically just copy-pasted. With Modern Warfare 2 I think Infinity Ward upped the XP to prestige to accommodate the larger number of challenges they introduced with the aim to keep a single prestige the same approximate length (i.e. number of games or amount of time played) as it was in CoD4.
With Black Ops I think they were trying to find a way to encourage more people to prestige in the first place by making each prestige shorter, and also trying to get more people to go max prestige by making total XP required lower.
Even though Modern Warfare 3 needed more XP to prestige my experience was that it took about the same amount of time as Black Ops because there were more challenges and a number of large challenges that were really easy (maxing out secondaries for example). They added 10 more prestiges post-release in two separate drops which I think was their next attempt to get more people to keep playing and keep prestiging.
My guess is that they realized during Black Ops' and MW3's years that the people who go to max prestige are highly likely to get all the DLC and keep playing throughout the year regardless. What they needed to do was give an incentive to the other guys, the ones who level up to 80 and then only play occasionally without getting the DLC, to keep playing more, which leads to Black Ops 2.
Is there a reason not to prestige in Black Ops 2? Not really. You don't lose any progress by prestiging, you're only kept from using certain items for a few levels until they're unlocked. On top of that there is actually a penalty if you don't prestige because you won't be able to unlock everything at once unless you go all the to 10th55.
I think They've found the right combination here. Those who are going to play the whole year and get all the DLC were already fine, they'll do it again this year. But those who would normally set the game aside after a month or a few months are more likely to keep leveling, and if they're playing longer they're more likely to purchase DLC. It's a marginal effect on everyone, and if you can move the margin you can move more of your product.
In Ghosts you will be able to unlock up to 10 soldiers. You can customize each soldier's loadouts as well as their appearance. Each soldier will have its own level, so the more you play with one soldier the more you will level up that one soldier. You will not level up and prestige sequentially like in previous games. You can level up each soldier simultaneously through the levels from one to 60 and then prestige all 10 soldiers at once if you want. So there are still 10 prestiges, but each one is attached to a separate soldier rather than all 10 being attached to your overall profile.
