The St. Albans branch justified it? I'm not suggesting electrifying it by itself, but as part of an MML scheme. It would enable through services from Leicester to Oxford and such like, not to mention freight.
Given Wikipedia reckons St Albans Abbey usage is approx 220k passenger journeys per annum, whereas none of the intermediate stations on the Bedford - Bletchley line exceed 50k, I'd say yes the Abbey line did justify it and no, Bedford - Bletchley doesn't. (I didn't look at Bletchley or Bedford Midland figures as they would be massively distorted by London traffic).
Through Leicester - Oxford services would only happen IF EWR happens, having said that, I think you've used a bad example, since Leicester - Oxford isn't a major passenger flow - I'd doubt it's enough to justify electrification - and there is a practical alternative route i.e. Leicester - Nuneaton - Birmingham - Oxford.
Similarly, I don't believe it would particularly benefit freight given that freight can already use the Bedford - Bletchley line if it needs to - I don't think there is a lack of links between the WCML and MML in that particular respect.