I always freely give away my trade rights, along with usually an alliance, the only reason i woldnt offer an alliance is if that faction was a faction i was going to attack in the near future. Trade rights are only benefitial in certain positions, such as if you own a small island in the middle of the sea, trade rights to somebody is going to bring you alot of money. Even giving trade rights to an soon-to-be-enemy has its benefits, they might loosen their unit amounts on border due to your newfound kindness, or you might gain more from the trade than they do...

If they will gain more from the trade, then they are obviously at war with somebody else and need the money to run the war, so its still benefitial to you