Dave Montreal is a strong card, but very expensive, and as a result rarely makes the starting line-up. His ability is quite defensive but useful in every shootout as a legal hand is nearly always possible with his bullet rating.
One tactic might be to use this ability to protect yourself from taking casualties shootouts s that are unfavourable, while hitting the opponent with abilities like Legendary Holster or Shotgun.
In order to get him into play it might make sense to use A Piece of the Action or Recruitment Drive to make him more affordable. Max Baine might also arrange a discount.
He combos with the Town Hall to reduce his upkeep, but this is obviously quite situational.