Not having read Rise of Sin Tzu I can only speculate, but part of Devin Grayson's characterization of Dick tends to be that a) it doesn't take much for him to sleep with someone and b) the dude is HOT and very few people don't notice that. I don't fully agree with that interpretation of the character, but that's her view and he probably is her favourite character. *shrug*
Tarantula, IIRC, followed Nightwing to Gotham City during War Games, and Batman accepted her help because Nightwing said he'd take responsibility for her. Bruce trusts Dick, and they were in a position at that point where the bat-team needed all the help they could get. By that point she *had* killed Blockbuster, but I don't think Bruce really found out the details until after War Games ended.
I just find it very disturbing and rather unfair that people call Tarantula Devin Grayson's Mary Sue, given what Tarantula did to Dick.