I think Pinkie would do it so she can party forever without needing to stop.

Rainbow dash, Because it would be a great tip of the hat to The R-Dash 5000 from PONY.MOV

Twilight Sparkle: Experimental purposes.  
Applejack: Increased efficiency.  
Rainbow Dash: Cool robot powers like rocket feet and lazers.  
Fluttershy: Shutdown mode for when things get too stressful.  
Rarity: Glorious customization options.  
Pinkie Pie: For shits and giggles.

You will be Upgraded, or you will be Deleted.  

@HJSDGCE  

 
He got the reference I was making ;)

e2258159rmqkr.net  
Wat?  
xD

@QuickFix  
Twilight…
 
Twilight, Twilight, Twilight, Twilight, Twilight, Twilight, Twilight, Twilight, Twilight, Twilight…

I think they’d all be willing to do it to fight an ancient evil in the past, but their pony bodies couldn’t go back in time so they’d have to be put into robot bodies. Applejack would keep her hat, of course.

Twilight as a brain in a jar is actually a thing.

Pinkie.  
Cause why not?

Pinkie would probably want to become a Barbobot for her own amusement.  
Fluttershy could probably be convinced if she were allowed to sit, motionless, on top of Pinkie’s body (though there may be a brief naming dispute between “Barbobot” and “Magnolia Prime”).  
Applejack might be enticed if the conversation were directed away from the artificial nature of it all and instead toward how she could now have the strength of five gorillas.  
Rainbow Dash could probably be talked into it if she were told she could have a robot alligator body.  
Rarity could be convinced if she were allowed to have a smaller, more petite body than the “Barbobot”.  
As for Twilight, she could be talked into it if all her friends were also given robot bodies, and she could likely settle for having her brain just lay exposed on top of its robotic “shell”.

Rainbow Dash because it’s awesome!

Twilight: Because she didn’t think it through. She also did it herself.

I both thank and appreciate the sealab reference.
 
Twi-For science  
Dash-for speed  
Pink-to party more efficiently