Two things concerning John McLaughlin in the late 1960s spring immediately to mind. First of all his very solid grounding in R&B (with Alexis Korner, Georgie Fame etc.) and commercial studio work - which must of itself have been of great appeal to Miles. Secondly his incredible stylistic versatility and ability (almost chameleon-like) to fit into numerous 1960s UK jazz contexts. Really everything from straight ahead mainstream (e.g. Sandy Brown's 'Hair at Its Hairiest') through to Kenny Wheeler 'Windmill Tilter' (also on Fontana) and Gordon Beck's 'Experiments In Pops'. On each of these, 'Johnny' McLaughlin plays a very key role in the success of the sessions.
I suspect also that his willingness to be 'moulded' by Miles and his music (along with all of the plaudits from Tony Williams) played no small part in his eventual selection.
Spedding also had a superb rock/commercial grounding prior to great work on such sessions as Mike Westbrook's 'Love Songs' (his work on this particular album is outstanding) and Neil Ardley's 'Greek Variations', for example. Less of a 'mouldable chameleon' though, I suspect, in the jazz context. It would have been fascinating to hear though !