Currently waterproof membranes are (mostly) implemented in three different ways:
Waterproof on the outside: will not be so great when you need airflow – waterproofing layers block out most of the wind. Additionally, we have not come across any affordable fabric that is waterproof + abrasion resistant. Most fabrics, like Cordura, require the waterproof layer to be fused to them. Most of the apparel which uses this technology will feature sparse ventilation to ensure the waterproofing is not compromised
Waterproof layer built-in: again, most of the airflow will be block out. The liner and not reach you skin will block the wind
Removable waterproof layer: requires you to take off or zip in the liner depending on the weather
We have been looking into this for a long time and it seems like for most people the versatility is more important than anything else. People want one jacket that can take them through all 4 seasons, or rather one jacket that can take them through any weather during a long distance trop. To provide that versatility you need removable layers.