Then why didn't she fend herself when Bowser kidnapped her?
Bowser kindnapped her BECAUSE she was the only one able to destory his plans of overhelming the Mushroom Kingdom. I think this was mentioned in the instruction booklet of Super Mario Bros. but I am not entirely sure.
@Warppipes:
Mario and Luigi can stand on them by putting on foot on every edge of the pipe.
How they work? My theorie is simple: Mario jumps into them, crawls through them, climbs them up at another place with his strong plumber-arms and -legs and then just climbs (or jumps) out of them.
The Piranha Plants construct a cork of roots to hold themselves on the same level and not falling down to the ground.