Usage and denomination may seem the same on the surface (which is why there is a peg in the first place), but a pegged currency is almost by definition not the same as the currency it is pegged to.

Problem being, that pegs can (and do) fail for a variety of reasons. In the case of sidechains, there may be a flaw in the consensus algorithm or its implementation. The project itself may be abandoned. Future changes on the main chain could lead to incompatibilities. In the end this means keeping sidechain tokens will always add risk since sidechain security will always be lesser than the one on the main chain.