Hi Nicolas,
I agree and understand that in CRM transactions are allowed to save even with error, but the mandatory field error is something which contradicts its meaning if still the transaction would be saved. If i think not as a technical but as a process point of view if SAP has provided mandatory field for transaction (which was not there in GUI side) then they should take care of it as well.
I have never seen any client who ask to make some field as mandatory and still allows user to save the transaction without filling that field, all they expect is that SAP will take care of it automatically. And yes sap has provided a parameter to be used in SMOFPARSFA for webui, but since now webui is the only interface in CRM hence i think they should put that validation at badi level instead at UI level.
I feel this because we face issues many times when we are allowed to load data without validation on mandatory field (at UI level) using bapi or idoc and when user open data in WebUI (the only interface now) and try to save without filling that mand. field they get the error. I hope all the data entry should be consistent either from UI side or using badi's.
I had hours long argument many times with customer on these similar issue and it's still debatable, i don't know which side to go Customer or SAP 
Thanks,
Shobhit