Hi Lukasz,
the BOL layer is shown in all the Web UI architecture diagrams as part of the UI. However, in my opinion this is misleading. The BOL is an implementation of business objects, abstracting the underlaying APIs. Therfore, I see the BOL as part of the application / business logic layer.
As a consequence I see absolutely no problem in using the BOL in custom dsvdoplments not related to the Web UI. In fact I require the usage of the BOL it in my team for all custome reports. Only if valid reasons exist, we don't use the BOL. An example for such a reason is if no suitable BOL object exists. in contrast to that performance is no excuse for not using the BOL.
Christian