The problem is that any moisture on or in the gun can't evaporate, so rust becomes more of a problem. The same applies if excessive lubricant has been used - it's got nowhere to go, so it's likely to migrate to wood grips and soak into them, to their long-term detriment.
However, if the gun is clean and dry, without excessive lubricant, and is stored in a climate-controlled environment where humidity doesn't matter, I don't see a downside to this.