Why Summoner necromance guides tell me to spend 20 points in bone prison?

Summoner is really easy to maximize in terms of summoning skills. Mine has full on RS, SM, 1p on decrep, AD, IG (have some items to maximize IG during the summon process; then I invested rest into Bone tree. Max BW, BP and 1p through BS. Some on CE to increase the radius. This way I can join the war with BS. Decrep for every boss. Merc has Pride, I made IG from Hustle weapon. They rock!