Why does anyone need an ETF to hold TIPs ?
ETFs do offer superb diversification at low cost. However, since TIPs are all from the same issuer, the US Government, there is little diversification benefit to be gained from holding an ETF. The only diversification is maturity diversification, and this can be solved by buying a handful of individual TIPs bonds with differing maturities.
Even so, since TIPs are only efficient when held in a tax-shletered retirement account, it makes since to hold TIPs with the maturity of the retiree.