Bikers don't get benefits from the grots and neither do Mek Gunz.
The first is expensive the latter dies easily to most meta options thus giving up ITC points for kill and kill more.
Lootas are not broken but are gate keeper units. They can do good but take A LOT of CP to work, as stated 1-5 CP a turn in a faction thst may get one CP back a turn IF they do some shenanigans like I do.
What makes lootas the better choice is two things:
1) consistency in damage. Being 2 damage in shooting is HUGE as a few saves can drop vehicles down a ton.
2) stratagem support. Shooting twice, shields and always a 5+ to hit and proc dakka is vital but expensive and limited.
Lootas otherwise die so easily especially if you catch them with out shields near