The polyurethane foam decomposes and the offgasing chemicals react rather badly with the dice and will eat into them. Adhesive tape and stickers left on your dice long term will also eat into them. Polyethelene foam "ethafoam" is suposed to be "archive safe" for 20 years and not decompose. Contrary to popular belief plastic is not forever and most start to come apart in 5 to 20 years depending on temperature and sunlight exsposure. Polyethalene lasts a lot longer than other plastics (all your plastic soda bottles are made of it).