its cheap. not everyone has enough gp for onyx bolts. and yes, bone bolts are cheaper, however you gotta be range28 before you can use it. bronze can be used from lvl1.
i think its closer to 8 lines at piscatoris. =) however its usually quite dull there. anyway i think its better to start next to the bank, then you'll be running bank empty, rather than with a full load of logs. saves energy. doesnt matter much if you have high agility.