He doesn’t need Imu lame powers

Controlling Xebec would mean that all Holy Knights including Saturn would help Xebec against Roger pirates and co. And Garp wouldn't have the role to fight the WG. Otherwise Imu would let him killed after the event.
Xebec is strong and probably show off what the Davy family was off. We gonna see a glimps of the Davy Family power which Teach show later in his battle.
Pretty sure there is another case why Garp and Roger teamed up against Xebec.