is your 15th sideboard slot worth more or less than being able to pay 3 once a game to draw a card? i think so, if it was drawing a random card from your vintage deck. if that card is a 3-5CMC creature and your deck has constraints, that may change the answer. but i think for the generic form of the question the answer is yes, but likely not for the specific questions.
The answer to this will trend more and more towards yes as more companions or frankly other cards on this nature get printed. Imagine if it is viable combo piece or something. Would you pay 3 mana to get a bad fireball out of your sideboard once per game? If you have infinite mana you certainly would.