It is important to remember that ‘dry’ oligos always have some level of moisture present, and are never actually completely dry. This is one of the contributing reasons for why resuspension in TE buffer (10 mM Tris, 0.1 mM EDTA, pH 7.5–8.0; for example, IDTE) provides a more stable long-term storage method compared to oligos stored dry.
However, oligos stored dry or in non-DEPC treated water, are very stable for the short period of time it takes to ship an oligo. IDT ships its oligos dry to provide the researcher complete control over the reagents they want to use for resuspension.
Please see the Technical Report, Oligonucleotide Stability Study
for data on oligonucleotide storage and a more thorough explanation.