If I teleport to get Merc closer, it's usually for tanking purposes: large group of cows, cursed, defiance not active, etc. Extra teleports are also done for herding purposes- one disadvantage of Sin compared to Necro is that revives help with aggro radius when they spread out. For medium-long distance, it's faster to bring the Sin to the cow vs lobbing the FB. If anything, I use defensive Merc to try to lessen teleports by not having to teleport out of small-medium threats, and also to lessen the chance of having FB interrupted. The DPS difference between Might and non-Might Merc is at best one FB, or 9 frames. Could be less on average due to low damage rolls. So the question is if Defiance can save 9 frames per group through better posturing and delivering FB quicker. It's also nice to play a tanky character, which was something I really appreciated when I played Necro afterwards.