Except, of course, that if it moves, it cannot not fire (then and now). And it cannot march either. There certainly can be occasions, where moving the HBVG is warranted, but not really to keep pace with the infantry.
That said, in 6th (and even more so 7th), the points of an Engineer would have been completely wasted on a HBVG. I took him with a Hochland, joined to a Mortar, to re-roll a Misfire on the Mortar, if needed, and otherwise snipe characters with the Hochland.