Depends how you play more than individual character needs. If you rely on luring enemies onto your highest defence/resistance units, advancing everyone slowly and steadily, sticking it on movement 4 foot Knights will help them keep up and thus get into more fights- you want them at the front.
On the other hand, boosting an already fast unit like a Falcon Knight with a rescue stave can create a great way of getting a wounded unit out of trouble anywhere on the map. Putting on a highly offensive unit (lords, Knights etc) will let you pick off threats before they hit your main line, and putting it on a dancer will also help them not gradually get left behind. Any healer on foot benefits from it too, as they are often slower than virtually all enemy units and most of yours too.
I tend to look at things in the view of creating a rounded, strong army rather than monster individuals. The former spreads xp around and stops over-reliance on the latter, particularly handy on siege defence missions where you need to fight at multiple choke points. Still, overpowered heroes are still useful for soloing tough situations and bosses.
