Hi Hasan,
IC web framework is totally different from WebUI framework as system saves the data from buffer on end of interaction, in IC web environment ideally user are not required to save the data hence customer never put any field as mandatory and not even advised unless it is driving some process.
The issue in WebUI is that you should not just set any field as mandatory and then choose not to fill it and just save it (this should not be taken as example of transaction saved with error in CRM) , the best practice to make any field as mandatory is that if it is driving some process then only it should be marked as mandatory and it can be done for any object in CRM not just the transactions (incompleteness procedure works only for transactions) and by doing so customer expect that those check should be taken care by the application itself.
I know in CRM transactions are allowed to save with error and that's what it makes different from ECC but the mandatory field error is what that is contradicting itself if allowed to save, think as customer point of view.
Regards,
Shobhit