Hi,
Once you added the value attribute to the model node the node became a MIXED node and teh GET_CURRENT() on this context node will return entity of type cl_bsp_wd_mixed_node.
Since this is standard component and mostly in all the method of IMPL class, CN class, like DO_PREPARE_OUTPUT, GET, SET, GET_I, GET_V etc are using standard code which has the entity of type cl_crm_bol_entity the assignment fails and exception occurs.
To fix this you will probably have to change in all places where cl_crm_bol_entity is used for that context node, which is extensive.
You might want to consider creating a whole new value context node with that value attribute in that view instead of adding that value attribute to the standard model context node, if at all you need that value attribute. What is the purpose of that value attribute?
~Anupam