They never made sense in the old or new system.
Let us examine the case of a currency issuing S&L which pegs to the dollar by fixed ratio. Since the pit boss has already declared the constant ratio, the buyers and sellers of dollars at the S&L simply see the value of the dollar plus the uncertainty of a simple, fair traded buffering pit. There is no third party, the pit boss cares not a whit. So we get a very accurate definition of the dollar in the new currency, measure to a slightly less precision than the dollar measures its own liquidity events.
But,absent the third party to manipulate, there is no theoretical, or software difference from using the pegged currency vs the original. Both operate under priceable constraints, meaning'; use the original dollar suffer currency risk, use the pegged coin, suffer the same risk plus a fair traded latency uncertainty.