Zurai said:
Your archer had a kobold in every adjacent square? If not, a simple Move action to shift followed by a Nimble Strike, or an Evasive Strike without the shift, would have gotten him out of trouble without provoking OAs. Alternately, she could have simply delayed her turn until after one of the kobolds was killed (if there's 8 of them, they're gonna be easy to kill) and acted then. In neither case would Defensive Mobility have helped, because you wouldn't even have had the opportunity to attack her.
These are fine options, but Defensive Mobility allows the ranger to use more offensive, high-damage abilities, instead of relying on low-damage strikes to avoid OAs.
Additionally, there are times when a ranger would prefer to use a full move action instead of merely a shift. This is particularly true when a defender is nearby and able to lock down the target to prevent re-engagement with the ranger.
While it certainly is possible for the ranger to use Delay Action and hope for a rescue attempt before shifting away and firing, I can see plenty of reasons why a ranger would prefer not to do that, and would rather shoot now and risk the OAs, especially if he has an automatic +2 AC due to Defensive Mobility.
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Strictly on paper, I can see why TWF is seen as the better option. However, a DEX-loaded ranger who is optimized for bow fighting simply isn't going to benefit from the ability to wield a longsword in his off-hand, so really it comes down to Toughness vs Defensive Mobility. This question comes down to a couple of things:
(a) Is the DM using tactics to try to knock out Strikers before Defenders?
(b) When engaged, does the ranger prefer to "run and gun" and put more distance between him and his opponents?
If both are true, then Defensive Mobility comes into play. This is especially true when the ranger is an Elf with 7 move, and can use that movement to put enough distance between him and his opponent that they CAN'T re-engage next turn.