Look at the morning and evening peak, where there are quite a number of services going MKC-LBZ-EUS or vice versa, it is these that are proposed would stop at Bletchley.
One of them, the 1816 from Euston, carries a large Bletchley load (at least 50) changing at LBZ onto the Southern service (or at the moment the 1806ish stopper off Euston behind it).
Using the evening peak (4pm-7pm from Euston) as the example - "quite a number" that stop at Leighton Buzzard and not Bletchley actually means
17.16 ex Euston
17.49 ex Euston
18.21 ex Euston
So 3 trains.
Whereas the following skip Leighton Buzzard but stop at Bletchley having stopped at one or other of the intermediate stations
17.05 ex Euston (stops at Cheddington and omits Leighton Buzzard)
17.30 ex Euston (as above)
17.51 ex Euston (stops at Berkhamstead and omits Leighton Buzzard)
18.30 ex Euston (stops at Cheddington, omits Leighton Buzzard)
Assuming the AM peak is similar, Bletchley does far better out of the deal than Leighton Buzzard does, despite Bletchley's population of 33,000 and Leighton Buzzard's population of 37,000.
And yes, I know Bletchley also serves some of southern MK, but Leighton Buzzard also serves a number of other places as well.