The reason why a lot of MW players have 2 bonuses it’s because you do not lose your main stats (versa + mastery) so you have nothing to lose, and a 5% essence font buff is better than nothing, even if you press this ability one time per week in some 3v3 for fun
There is one main reason why nobody plays with 4 pieces:
The bonus is just bad, it gives you 450 extra heals which is nothing, because you lose a lot of mastery (because stats from the class set are trash). Mastery gives you extra heals from mist as well, so it is more profitable to play with PvP gear that gives you more mastery. With 4 set bonuses you will have around 115% mastety + 450 from the set bonuses every 30 sec. With full pvp gear and 2 set bonuses you have around 135-140% mastery, which is around 4800 additional extra healing from mist, and its stable. So now ask yourself what is better: having 450 extra healing every 30 sec with less mastery and less versatility, or 4800+ 24\7 with a lot of versa and mastery