Memory that is free will upon allocation either be over-written (e.g. DMA from HDD/network) or zeroed (e.g. stack frame) prior to use.
Making this the default behaviour will add a performance cost in the over-written case. Part of the cost could be hidden though by having
a low priority memory clearing thread which used the appropriate instructions to avoid filling the data cache.
The extent of the cost can only be determined with proper benchmarks though. Then whether the cost is acceptable is another question...
Disclaimer: I'm not a kernel hacker so I could be full of it.