Yes. I wouldn't for a moment argue with your point that pilots need to understand the limitations of ACAS, and that's particularly relevant to using it to manoeuvre in the horizontal plane.
I think there are three reasons why ACAS uses the vertical plane for resolution:
1) greater precision of the measured value (and trend) of level vs azimuth
2) easier 1-dimensional deconfliction problem in calculating an advisory
3) ATC tends to use horizontal deconfliction (vectors) so it's less likely to contradict ATC
All of those are reasons for preferring a vertical manoeuvre if possible, even as a human-initiated response to a TA. If it's not possible, using the azimuthal trend information to increase separation make sense.