Sorry for 1) the double post (first was in the wrong forum) 2) my bad Englih.
As a professional editor I think 3 point editing is a key feature.
A NLE where you can't do 3 point editing can't pretend to be use by professionals.
You can imagine other logic but : 1) why reinvent the wheel 2) the editors love 3 point editing 3) (for the moment) Other logic are slower and less permissive.
So, what is 3 point editing ? I'll try to explain but it's not easy for me in English.
This is the content of my source and my timeline (for the examples, I will use only one layer), each letter in a frame :
SOURCE | V1 | 1234567890
TIMELINE | V1 | ABCDEFGHIJKLMNOPQRSTUVWXYZ
Case 1 :
Now, i put a mark IN and a mark OUT in my source (to say, i want this part of the rush) an Mark IN in the timeline (3 marks -> 3 point)
SOURCE | V1 | 123 i 456 o 7890
TIMELINE | V1 | ABCDEF i GHIJKLMNOPQRSTUVWXYZ
Now I have 2 solution (corresponding in 2 buttons in the interface or 2 shortcuts in my keyboard).
a) I press REPLACE : my selection of source go after mark IN and replace the same length.
TIMELINE | V1 | ABCDEF-456-JKLMNOPQRSTUVWXYZ
a) I press INSERT : my selection of source go after mark IN and everything after it move.
TIMELINE | V1 | ABCDEF-456-GHIJKLMNOPQRSTUVWXYZ
Case 2 :
SOURCE | V1 | 123 i 456 o 7890
TIMELINE | V1 | ABCDEF o GHIJKLMNOPQRSTUVWXYZ
REPLACE :
TIMELINE | V1 | ABC-456-GHIJKLMNOPQRSTUVWXYZ
INSERT :
TIMELINE | V1 | ABCDEF-456-GHIJKLMNOPQRSTUVWXYZ
Case 3 :
SOURCE | V1 | 123 i 4567890
TIMELINE | V1 | ABCDEF i GHIJK o LMNOPQRSTUVWXYZ
REPLACE :
TIMELINE | V1 | ABCDEF-45678-LMNOPQRSTUVWXYZ
INSERT : NO
Case 4 :
SOURCE | V1 | 123456789 o 0
TIMELINE | V1 | ABCDEF i GHIJK o LMNOPQRSTUVWXYZ
REPLACE :
TIMELINE | V1 | ABCDEF-56789-LMNOPQRSTUVWXYZ
INSERT : NO
I hope you will see all the advantages of this logic: rapidity (just one click or key is quicker than drag'n drop), precision (I know exactly what's happens and why) liberty for the editor (for the moment, in kdenlive I can't put a clip between two).
I know it'smust look quite complicate comparing to just drag'n drop. But you have both.
To me it's like "styles" in a text editor : you can just use bold, italic and font size an think it's ok. But when you have a lot of work to do you need something more productive.

I try to get it exactly, tell me if I'm wrong...
- Normal editing
Like now : the editor has to find a free place in the timeline, at the end of the editing or in another track.
This could be cumulate with a real overwrite mode : the editor has to find a free place in the timeline OR can replace some frame with the new clip.
- Overwrite :
Do you mean we replace the entire clip ? This could be funny for beginners but not exactly what's editors need.
I try to give a precise example. I have an editing with 2 clips A and B :
Timeline V1 : AAAAAA i AAAABB o BBBBBBBB
In my player I have a clip C.
Player V1 : CCCiCCCCCCC
When I do Overwrite I don't expect to have V1 : CCCCCCBBBBBBB but V1 : AAAAAACCCCCCBBBBBBBB.
I have overwrited my timeline zone with the new clip. Is this what you mean ?
- Insert : Look good.
Track selection with arrows : It can work, it unusual but can be try. Normaly you are more productive when you can chose separately in witch video and audio you put your new clip.
Timeline zone : great ! Thanks you ! Now we just need the good insert/overwrite function.
Insert selection at timeline zone in point, insert selection at timeline zone out point, insert selection in timeline zone : we just need the first (second can be try, third is actualy an overwrite, isn't it ?)
If you speak french we can talk by mail or by phone, it will be much easy for my. (French are terrible to learn, foreign language).