When you get liposuction of a problem area, you are removing fat from an area that has disproportionate amounts of fat deposited . After liposuction, you no longer have that one area of extra fat - you just have the same amount of fat everywhere. If you do gain fat after liposuction, it will be evenly distributed. The problem here is two-fold.
First problem: Before liposuction, gained weight would mostly go to your problem area and nowhere else. Now that your fat is evenly distributed, if you gain fat it will go everywhere evenly and this includes areas that you didn't gain weight before.
The biggest problem: If you are used to gauging your weight on the size of your problem area, you will get into trouble because as it gets bigger you are getting bigger everywhere. By the time you see your problem area back to the way it used to be you will have gained alot of fat in other places!
Example: If you have a problem area of the outer thighs and you get liposuction of the outer thighs, your pants will fit better and you will be happy. If you gain weight, but your thighs don't look bigger, you may not notice. If your thighs would have been pushing out those pants at 5 lbs of weight gain you may notice the waist getting tighter before you realize that you've gained 15 lbs!