Well, I had kind of a same discussion in my dungeonning group, and the leader told me this :
* march april and may to farm Huntsman in MoS
* may and june and july to farm BRP motif
* july and september and october to farm one or two wrathstone motifs

As a (not so much anymore) new player, I also have the difficulty to get that. It could like a strategical move from the team, but I see this as a wrong move. Instead of uploading a DLC with all its content, motifs included, it's delayed, like "we still have bugs and not everything is done but get the main stuff and you'll get corrective patches and motifs later".

And if there is a strategy behind, I then don't get the Murkmire motifs. DeadWater was available directly, and ElderArgonian is already out. Wolfhunter was published earlier, and six months later we are still waiting for.