I tend to favour the notion of his being a cold-blooded pragmatist with a tendency to throw ice water on warm, fuzzy Southern illusions (Hence his having been landed in the historically inconsequential office of Vice President*). This also ties into the notion of Wilson being (relatively speaking) a 'kinder, gentler' Whig, since the Old Guard might want a man of their own on the ticket (and Wilson might want to make sure the man in question is somebody he's in no great danger of being shunted aside for).
*Note that this is partly inspired by my mental image of him as a sort of mirror image to Theodore Roosevelt (Who in our own history seems to have been selected as McKinley's Vice President mostly so he could be safely shooed away from the levers of power ... in theory).
The idea of Wilson having a conservative vice president to balance the ticket makes sense though it does raise the question of how Semmes was able to get the Whig presidential nomination in 1915 if he wasn't ideologically aligned with Wilson. (I would certainly expect a term limited Wilson to move heaven and earth to insure his hand picked successor ends up in the Gray House come March 1916.)