The weld zone will be chrome depleated and as shoota said the dissimilar metals is an issue. No worries if it's painted but once the two metals are exposed you have a galvanic cell and accelerated corrosion. It's to do with the galvanic series of metals more reactive metals essentially sacrifice themselves for the less reactive (sacrificial anode). The same reason alloy heads corrode against the block.

im not to sure about that one mate i know what you mean but if its primed properly why wouldnt it last longer if your only replacing little square sections at a time

You can weld mild steel and stainless together but I believe the two different metals react with each other and cause corrosion between the join. Same if you say put a stainless washer against raw steel.
You could make the whole car out of stainless and avoid the problem!
Stainless is also heavier as it is denser than mild steel.
I'd like to see a fully polished stainless Datto, very nice!

hey guys just wondering why when i see alot of rust repairs done why people dont weld in stainless sheetmetal instead of mild steel? would last alot longer