The discussion is becoming a little clouded reference the original question. That was why are there bugs for V2 & V2+15. That is to do with allowed manoeuvrability i.e. bank angle. Climb performance on 1 or 2 engines is another issue. The FD pitch bar will give guidance on that. If the FD is off and you have an engine failure then, yes, the bugs will be of significance. However, under normal departure ops the FD will be in view. You now have a choice of using LNAV or HDG SEL for routing, be it normal SID or ETP. If you are <V2+15 you need to use HDG SEL with 15BA max. That is the manoeuvrability issue. Climbing at V2-V2+15 is the obstacle and climb %gradient issue, which could be straight ahead or not. The speed has 2 effects, one lateral one vertical and they need to be understood individually.