If you travel to USA frequently then you'll have no problem just ripping it out and throwing it away. The process with I-94 departure cards is so silly that many are lost all the time. Failure to collect on departure only has potential to cause problems for you if you have no evidence that shows you did not overstay your previous visit. A return visit within the original visa waiver validity is sufficient, as is an entry stamp from somewhere else (possibly excluding Canada and Mexico).