It was done to make leveling harder after your prestige. For people like me who sometimes only focused on challenges I could gain levels extremely fast by doing so. Work on several challenges then pop them in one game gain 2-3 levels at the lower levels. 1-2 at the higher ones especially if dxp was active too. Hell with dxp and challenges you could go through an entire prestige in a weekend.
The downside is less will prestige now unless it is a forced prestige. For most wont want to waste/invest the time to level up slower because of this. Unless there are some challenges that only unlock after prestige. Otherwise waste of time to prestige and level up again.
The idea was probably to try and force attention back to the game play and less about challenges after the prestige. Which also makes boosting less effective.But for those that challenges were the fun and main reason to play they now have less reason to ever prestige.
Only thing may be is if each custom soldier gets their own challenges to work through each time they are started. If it is a global set then not much of a reason to even make a second soldier once the first one is done and challenges completed.
So basically it will be for me complete one soldier to prestige level, do not prestige at all. Then start on next one and repeat. Once all 10 are done trade in game for something else. Probably FM5 or Destiny or TitanFall or BF4 which ever I don't have at that time or something else entirely at that point. No reason to continue playing once all 10 are at prestige ready level.