The main rationale behind IEP for the fast Cambridge services is to allow them to keep up with the IEPs on the East Coast on the common section to Hitchin.
I agree that this is a much better offer than Thameslink which is likely to end up with very high density units ie small numbers of seats and lots of standing space to achieve the dwell times necessary in the central section for 20-24 trains per hour.
A couple of years ago, I went to a lecture about Crossrail. The person giving the lecture, I think Rob Holden, said that Colchester to Oxford gave better financial results for Crossrail than the option chosen of terminating in the east at Shenfield. However, the mix of trains and trying to run an outer suburban service through the central section mixed in with the inner suburbans made it operationally impractical in their opinion and the idea was dropped.