my reason to use then is a very small point of contact on 3D surfaces, giving high "resolution" for lack of a better word, ...while also benefitting from a thicker shank on the deep router offset cuts, like routing a 1-1/2" thick HDU plaque, the long 1/8" tapered ballnose can rout a real nice faux woodgrain background, but cut through the 1-1/2" material as well, without a tool change, plus getting down to the depth where I want the woodgrain could be more stressful & requiring more passes & more time with a 1/8" shank...
regarding a straight flute... if I'm not doing a textured surface, I would never use a ballnose, tapered or otherwise... (but I'm still a beginner too... so maybe I'll learn some other things here myself)