Receptor Tyrosine Kinases (RTKs) present themselves on the surface of a cell as monomers until a ligand (such as a mitogen) binds and causes the formation of the active dimer. However I do not understand why truncations are so commonly seen in retrovirally delivered oncogenes or in oncogenic mutations. My confusion here is that if mitogen binding of RTKs facilitates the close proximity of two monomers, then simple overexpression should allow for increased transphosphorylation of monomers, and therefore be mitogenic (which it does). But then what advantage would a truncation that removes the extracellular portion of an RTK have in increasing transphosphorylation of monomer units?
As you said the simple overexpression can lead to transphosphorylation and activation of mitogenic signalling. This helps the cancerous cells to survive.
By truncation of the molecule the cell also gains a benefit, because it could prevent inhibition of the receptor from outside the cell. The truncation furthermore could be induced by treating cancer with inhibitory antibodies against the receptor. For example a treatment with anti-EGFR antibodies could induce resistance by mutation and selection. One of this mutations could be the truncation of the receptor.