For me opera is one of those complete art forms that is only truly successful when all aspects of it - music, singing, acting, staging - are done well. So to answer your question, I find acting to be a pretty important consideration, in two separate ways - physical movements on the stage, as well as acting with your voice.
Physical acting on the stage enhances the drama, the story, and makes watching opera a more complete experience.... I do agree however, that it is frequently overdone, made too melodramatic and hammy, and then it does get bothersome. Truly good actors know when less is more, which is quite often, really. I suppose I appreciate it so much because of how rare it is to find an artist who can successfully integrate good singing and good acting - its an incredibly difficult thing to do. This is also why I am more inclined to like an artist who tries to integrate the different aspects of being an opera singer more than an artist who has perfect technique but practices the arm-waving school of acting.
However, for me, more important than physical acting, is acting with the voice. Listening to an artist who can give insight into a character's emotions or frame of mind using simple differences in coloring/tone/ornaments can be an enlightening, marvelous experience. A simple example is Callas singing Dite alla giovine, where she inserts a slight, almost imperceptable pause before the word 'pure'. Its such a simple, uncomplicated detail, yet, its effect, dramatically, is simply
That's my take.