Due to the microstructure that forms when welding CS to SS with CS filler metal, the weld will be fairly brittle. On some applications this can lead to cracking during or some time after welding has taken place.

Elaborating a bit on what GRoberts said .....

The issue is dilution. If you use carbon steel filler, it will mix with the stailess steel metal and the result will be a high alloy ferritic steel with high hardenability, and in this case, your room temperature weld metal will be likely essentially 100% untempered martensite. This is brittle material and susceptible to cracking under impact load, or at stress raisers, or due to hydrogen input to the weld metal from the weld process.

If you use the 309 alloy filler, which, it may be noted, has elevated nickel compared to the 304, or 308, alloys. The elevated nickel in the 309 filler helps to ensure, under typical dilution conditions, that the room termperature weld metal will be austenitic, just like the 304. The austenitic microstructure is ductile, tough, and not susceptible to hydrogen cracking.

Also, dewa search and you will find much more on the topic.