TLDR is as follows
Relic shield gives 20 more HP and 2 less AP, no champ in the botlane has high enough scaling in the early game to worry about 2 AP in lane and the 20 Hp realistically is a free minion or auto or so that you can tank before death.
Sitting behind the wave with your ADC and occasionally autoing a cannon or melee minion is MUCH safer than going in for trades against the enemy botlane and both of the items produce the same amount of gold at similar rates meaning that the relic farming is in most situations going to be the better option. Even if you think you're going to win the lane its likely worth taking as a precaution in case you don't.
You can push waves, spelltheifs can't.
you're not forced into expending mana on trades as you are with spellthiefs which no longer gives mana regen.
feel free to discuss