Why are truncations in Receptor Tyrosine Kinases so sommon in transformations?
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.