I don't blame Bay for not introducing a real Pretender into the movie, because it would require either:
a/ Mass shifting; as seen in the G1 animation; Pretenders were able to grow to Transformer size before phasing out of their shells, then shrinking down to human size when inside their shells.
b/ Gigantic humans; as seen in the G1 Marvel comics. The Pretenders were only able to pass off as human(oid)s when they were on places other than Earth where other giant humans lived/frequented such as Grand Central Space Station, Femax, VsQs, Pz-Zazz, Cheyne etc.

But for the live action movie; being set on Earth and where mass shifting would look stupid, I can understand why Bay didn't go with a true G1-style Pretender. But in modifying/adapting the Pretender concept for the movieverse he has inadvertantly created - at least in my eyes - a beast moded TF rather than what is traditionally considered to be a Pretender.