Weakness????

If You plasma or torch cut quenched & tempered material, the heat affected zone [HAZ] may end up softer. The HAZ from welding may have the same effect.

When You excede the temper temperature, You soften the material.The only way to restore the temper is to anneal, quench & temper the same as the manufacturer did.

Dave is on the right track. Plasma cutting does not necessarily weaken the material. It depends on the material. In many instances the material effected by the plasma regime will be hardened and therefore strengthened, if also embrittled, or with a loss of ductility, somewhat.
So bottom line, its all related to the specific alloy as to how the HAZ of the plasma regime effects the material. And how the material is 'recovered' will also be alloy specific.
To make it even more complicated it is not only alloy specific but can be related to what region of the HAZ you are talking about. as you get farther away from the heat of cutting the properties of the material will change.

yup the posters above nailed it.

Also the HAZ of a plasma cutter is relatively small. if you weld over a plasma cut, your welding heat affected zone will be much larger and basically "erase" any evidence of the plasma HAZ