In light of the SR111 crash, the procedures which have been recommended to all pilots is that if you have a smoke in the aircraft from an unknown origin, land the aircraft asap. I am not going to discuss the particulars of the Swissair crash because the crew did excatly what they were told in the checklist, however once the CVR's were reviewed, it was understood that these procedures cost valuable time.
Today, the understood rule is that if you have smoke, and you don't know where it is coming from, land the aircraft, IT WILL BE SAFER. Certain aircraft do have Max landing weights, but the chances of a gear going through the wing are quiet slim, as long as the pilot doesn't try to bounce the aircraft down the runway.
In regard to the Air France crash, the piece of metal on the runway (it is assumed at this stage) burst a tyre which sent peices of metal/tyre through the wing, bursting one of the fuel tanks/lines to engine 1/2. The leaking fuel was ignited by the afterburners, burning through the wing, cables and control surfaces. At the same time the engines were being starved of fuel which was being burnt before it reached the engines, thus power to 1/2 was lost. Coupled with limited flight controls, the aircaft started to pitch up and without 2 engines, stalled and crashed to the ground. (this has been the generally accepted version of events, however any further editions are very welcome)
Aircraft I've flown: A300/A310/A320/A321/A330/A340/B727/B732/B733/B734/B735/B738/B741/B742/B744/DC10/MD80/IL62/Bae146/AR