I don't think even bad drafters do that (which Grigs certainly was). Given the amount of control you get out of a rookie...it wouldn't make sense. But it doesn't really matter because those other positions (S, ILB DT) were long-term needs as well.
I think good drafters draft for a combo of value/talent (however you want to define it). If a top talent falls, he becomes a great value, which makes him desirable even if he doesn't fit an immediate need. If the talent level is similar across two players, the player that fits an immediate need and can have an immediate impact becomes the better value. You also have positional value to consider. A good starting 3-4 DE is worth more than a good starting C for example...because of the surplus value in a rookie contract (think having Wilkerson and signing a C vs. having Kelly and trying to get a DE in FA).
But Dorsett didn't really fit into any of those categories. He wasn't demonstrably more talented than other guys available and he didn't fill an immediate long-term need for a Super Bowl contending team.
And given that he was going to be the WR3 (and not WR1 or even WR2) on this team, his positional value was low because he wasn't going to give you much surplus value (WR3 production is readily available in FA and the later rounds of a draft). The Colts were going to extend Hilton and Moncrief was being groomed to be the WR2. They also had two TEs and Andre Johnson for at least one more year. Dorsett was just never going to give them the production of a 1st round pick.