This is one of the most ill conceived and ill informed opinions i have read in awhile. All of your reasons are based upon what you perceive may go wrong based upon assumptions of things that are probably not true.
I've done extensive tests myself and observed countless others. That's the basis of my opinion.
You assume that standard length cue, standard only by convention, is the proper length for everyone, regardless of stature.
NO I DO NOT! If you are very tall, then extensions may be for you. I still think you'd be better off buying a longer cue to start with, but if that is somehow not a possiblity then an extension may be a solution. What I OBSERVE is people of perfectly normal height and armspan using extensions.
You assume that every cue, balanced as is, is perfectly balanced because that is how the cue maker intended it to be balanced.
What I observe by trying various cues with extensions is that the balance changes. This applies especially to butt extensions. If you have your hand all the way on the back of the buttcap, then maybe it won't matter as much. If your hand is in a normal position (back of the wrap for instance) then adding a butt extension may make the cue cumbersome, especially in those situations where you need to choke up on the grip.
You assume that all extensions, without knowing any actual specifics of build and location, will negatively alter the Balance of any cue.
I can say that ALL extensions I've tried have in some way changed the way the cue plays. Most have been very negative, some have been less so.
You assume that rail shots, masse, and obstructed shots are automatically harder using false logic based upon your own opinion, not actual proof.
I can only trust my own trial and error on the subject.
You assume that having a cue that is a more comfortable length will not improve the performance of a shooter on any shot, also without any actual knowledge of such.
I've NEVER seen anyones game change positiively as a result of adding an extension apart from very tall people with long arms. I've seen many people BELIEVE that this was the case, but then....
I have been playing with a four inch mid extension for two years now, and will never go back to a"standard" length cue ever again. From the very first shot I took with it, it felt natural. There is no shot that is harder due to the extension, in fact all play in general is easier because the length suits me much better than a "standard"length cue. I shoot off the rail and over balls with ease. I also masse and shoot half ball jumps with ease as well.
That's great.
It is intelligent to question convention in order to understand if that convention truly is the ideal for you. It is "really, really stupid" to blindly assume that what someone else told you was the ideal is in fact that. It is also "really, really stupid" to condemn things that you do not actually know anything about using your own assumed "facts".