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Are they even pin-compatible perhaps?
Yes.
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I suspect it will be some (minor) difference in power consumption and heat?
A little bit. Not a lot. It certainly doesn't mean a sub-1-Watt MPC5123.
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I mean, why else would anyone use the 5123 instead of 5121e?
There is obviously a small price difference to take into account (which makes a big difference if you order a million chips) as is with everything, and if you don't need 3D graphics (rolling screens in a kiosk or simple 2D gaming) then you needn't be paying for it just to turn the PowerVR core off.