Because I find this in SAP documents:
SAP recommends UI tech:
.... WebClient UI..
SAP does not recommend UI:
Here is more info on BSP from help.sap.com :
WebclientUI is basically a framework (and also a website) extended from the BSP technology. It represents the external representation of SAP's CRM. You cand pretty much do anything you can do with BSP (write custom HTML code and so on). You also get more custom extended HTML tags (such as THTMLB and CHTMLB which help you keep the same look and feel on new components that you create in Webclient UI ).
More info on Webclient UI as user interface and framework.