Scientists videotape fruit bats having sex: is fellatio useful?

The paper Fellatio by Fruit Bats Prolongs Copulation Time (M. Tan et al.; 2009) provides a groundbreaking look into the sexual lives of animals - by reporting on the observed use and effect of fellatio in the sexual habits of short-nosed fruit bats (Cynopterus sphinx). What we knew before going into the topic of this work is that fellatio is a common copulatory aid in humans, and that it has been observed as playful behavior in juvenile bonobo apes (I'm surprised, but I'm also not surprised). However, now we know that female-to-male fellatio is a common foreplay activity in short-nosed fruit bats, and that it brings some additional benefits to the table.
The issue with recording the details of bat copulation boils down to it most often occurring in darkness and in secluded areas or dens built by the males. To circumvent this unfortunate condition, the authors caught and caged 60 adult fruit bats and paired them up (males with females) in cages with nighttime video surveillance for several days. Their voayeuristic behavior brought results: they recorded 20 of the pairs mating, and over half of those instances included female-to-male fellatio. As the male's penis is in the female's vagina during copulation, on occasion the female will reach to the male's shaft to lick it. What was discovered is that the recorded instances of copulation that included fellatio lasted on average almost twice as long as those without fellatio, and that one single occurrence of fellatio prolonged the act for approximately 6 seconds! Thus, they have managed to document fellation in animals that may have functional significance.
The authors continue to provide several hypotheses as to why this is the case, but admit that more extensive research on the oral sex in fruit bats is needed. They underline that this is the first large-scale study of this type that indicates an evolutionary role of oral sex. If you cultivate further interest in this topic, a diagram of the act can be found in the paper, but if that isn't enough, the authors have graced us with video evidence.