Edward's Links
Alastair is a useful pawn. He has a talent for enrapturing the masses that Edward can't quite achieve on his own. He'd be formidable if he wasn't a spineless coward, but that just makes it easier for Edward to keep him under his paw.
Strictly business and nothing else. Alastair would do anything to not be seen with or associated with Edward, lest he lose his good reputation amongst the people. He would cut ties instantly if Edward weren't so useful in tracking the angel, or if he weren't so terrified of him.