Engineers and Bridges: Till death do us part?

[HirooOnoda]

I'd like to know how come engineers who are maintaining a bridge have to sit there and get slaughtered by enemy fire while they are in the act of dismantling it?

If mechanized and motorized infantry can ditch their vehicles to enter marshes and swamps, why can't bridging engineers ditch their bridges over a period of one turn? And if the engineers are disrupted does that mean they'll never be able to complete the dismantlement?

Hiroo