You work up the ladder. Never down. You don't release a murderer to convict a Burglar, you don't release a burglar for someone taking bets, you don't release someone taking bets for someone pick pocketing a candy at at the local 7/11. You work up the line.
Pmac and NE both good points also.
By that logic, they shouldn't have used Gravano (an underboss) to convict 7 Gambino captains, as well as several more Gambino soldiers/associates and captains, soldiers, and associates from other families.
Mods should mind their own business and leave poster's profile signatures alone.