If the company moves their 401K while you are unaware or out of the country or whatever, many SPDs are written so you are "cashed out" of your asset allocation, and put into the least volatile account, usually a money market.
So far we have had former companies change custodians. In the accounts that moved we have only had mutual funds and they were cashed out and put in comparable funds. Since there are no tax implications with the 401K it has gone pretty seamless in the past.
I would like to buy more TIPS eventually but those can be a bit risky if they needed to be cashed out before maturity, so I agree that is the one downside we have with the former employer plans. For now I have been putting the TIPS only in our own plan where we can control the choice of custodian. I only invest in stuff for the old 401Ks where we wouldn't take a beating if we had to cash things out if the custodian was changed.