I have been doing some reserch on this and back with some info from Boeing.
I have been told to understand this as a double use data imput. The design of these FMCs where done long ago and had to use as little steps as possible and have the least computation processes as possible. Since then we have been stuck with some design limitations but are solved in new models like B787.
In the early days where flights of the 737 where not that long and in the 757 and 767 wind predictions not that good a fast way to introduce a wind imput to the FMC was to insert an average wind in all the points of the route and this was the option. Climbs speeds at first would not vary at all and would not consider wind and only do so for CRZ and DES when using CI.
When oil price was no joke and companies staring to say that climb speed should be optimised climb speeds (about 200Kg savings possible) they started to take average wind and was almost right for short flights and basically an easy way to answer the clients needs.
Having powerfull CFP programs and very accurate winds the best option is to insert all CRZ and DES winds in LEGS and DES/FORCAST but renders the CRZ WIND usless for those phases. The idea now is to try and get a average climb wind from CFP and inster this ins CRZ WIND.
This is a thing we are trying with automatic wind loads via ACARS. The data input we will insert for that data field is going to be average climd wind. (hopefully)
What I do tell everyone is that inserting ALL your winds is very important. Many people don't insert DES winds and even though we all know that the recent VNAV updates are .... CRAP we have been doing trials and have found that a more accurate TOD point due to DES wind info being inserted can save an average of 50 - 100Kg. That is a lot of fuel ant the end of he day for a 300 fligh per day company. About 1.2 million €/year.
Good motivated and trainned crews can save easily 5 million €/year with precise fuel saving policies. That is what CEO and other narrow minded managers don't easilly understand.
Hope to not have said too many stupid things.