Sixth member
 

I have nothing against but is necessary to have a sixth member everytime ?

Probably from the marketing standards. It gives them the chance to sell another morpher, set of weapons and zord that they can feature heavily in the show [read: heavily advertise], so they can earn a tonne of money. Even Go-Busters had a second set of mecha and morphers, with Beet and Stag Busters debuting when a Sixth usually would. The last one that didn't really fit any of these standards was Go-Go-V, and even then they added one in Lightspeed Rescue.

Also, their characters are more interesting to us, as I've noticed from the boards. Due to their seperate nature and occasional conflict, they usually stand out more and become more popular characters than some forgettable members of their team. Their ofthen unique design can help with this. I can say that Akira (Tokkyu-6) is probably the most interestng character in that show, but all this is just my opinion.

It's an extra set of toys. From a marketing standpoint, yes I'd say it pretty much is.

The last two sentai couldn't have a sixth member was abaranger and gekiranger.

AbareKiller counts as a sixth, and Geki effectively had the GekiChopper/The White One in the same place- he had his own morpher and mech that made its own robot.

Well GekiChopper had an entirely new changer and a mecha, so toy line-wise I guess it did.

EDIT: Dammit Akiba stop ninja-ing me :lol

Hey, I'm excited for Nininger. I wonder what they're sixth is going to be... How long did it take for us to get news on Tokkyu-6

But abarekiller and gekichopper is not one of the sixth members of gokai silver.

Abarekiller and Gekichopper are not considered sixths in terms of line-up.

I say that it some times is and sometimes isn't. Like for instance toq6 was necessary for plot reasons such as Swartz and his rivalry and past and the Chou Chou Toq Dai oh combination.
When it's not necessary is in a series like megaranger or time ranger. Not saying timefire wasn't b.a and awesome but I just don't see why he was added into the series.