There is nothing much to say here, but I like filling in cards that haven't been reviewed yet. Obviously, this serves no purpose unless your deck has 3+ restricted cards (Venom is an exception...need 4+). There is only a handful of non-identity cards that are restricted. Most of these are actually pretty strong, so it's reasonable to include a Side Holster if they bring you above your limit. The heroes that have made me want to play this are Venom, who has Weapon combos, and Rocket Raccoon, whose identity set comes with 4 restricted cards. Be mindful that some restricted cards like Rocket's weapons or Sonic Rifle have charges, so you could be able to discard them before the restricted-ness hinders you. Therefore, consider if your deck would ever need to have 3 or more in play at the same time. This is a card you'd always prefer not to play since it does nothing on it's own, but it can enable some archetypes.