Is there any reason to base the mines?

Style Points, the most important of all points.

They have to have bases for you to use them.

The bases replace the footprint of the objective token, so yes you need a base for measurements.

3 hours ago, arnoldrew said:

They have to have bases for you to use them.

Not necessarily. Yes the bases are needed to mark contact when claiming a dropped bomb, but you could also place a un-based mine on an objective marker, which is the same size to mark this. This is the same in principle as using the objective tokens if you don't happen to have bought priority supplies for the based models. You definitely don't need based mines on the bomb carts as the cart gives cover independent of the profile.

5 hours ago, Welshie13 said:

Not necessarily. Yes the bases are needed to mark contact when claiming a dropped bomb, but you could also place a un-based mine on an objective marker, which is the same size to mark this. This is the same in principle as using the objective tokens if you don't happen to have bought priority supplies for the based models. You definitely don't need based mines on the bomb carts as the cart gives cover independent of the profile.

Correct, they are not needed for the cart. But they ARE needed while the bombs are claimed as well as dropped. Since model cannot overlap objective tokens, the base indicates were the models cannot be, helpful for knowing what models can get into base to base contact with the unit leader.