I think in any context it's loaded, because it speaks to what James sees Snape as being, at root. An ontological problem, if you will.

Problem is that Bella's one of the gang; and per OotP and HBP, Bella is older than Sirius (he says he hasn't seen her since he was Harry's age). JKR sucks at chronology, but not THAT badly. There must be a reason why Sirius says Snape was involved with this group who all went on to be DE's; he's been right about Snape knowing a lot of hexes and such. Is it so hard to imagine young Snape enjoying watching Bella fight with her younger cousin?

Lupin mentions that the spell was very popular.