I think you misunderstood how leves are going to work. You get 4 every 12 hours, yes, but those 4 "Slots" will keep building up to a max of 99 so as long as you have free slots you can get more leves. Don't do leves for a week and you will have 56 leves you can do whenever you want however you want.

It will be easier to level a crafting class because the leves will reset when you complete or fail them, not just when the timer runs out, so you can repeatedly get those annoying high ranking crafting leves that there are so few of.

The issue some are having is that we are getting a total of 12 leves every 36 hours as opposed to the 16 we have now. If you do only crafting or only battle/gathering leves, you are progressing much much faster, since any leves of the other type that you never used would essentially be up for grabs for the class you actually want to level. If you split between them you will be progressing a little slower than before.