Why it is so expensive I have no idea ask your Juniper salesrep! But it is expensive.
The advantage is that you first of a have one device! but more important is that you can terminate all the PVC's on
your SRX and fully utilize all the "protocols" you need to bring up a service. instead of being bound on some kind
of "dslmodem" running as kind of media converter and you need to bridge everything between the PVC comming in on the dsl connection towards your SRX.
Also most of the time with using a "dsl modem" in "bridge" you can only bridge one of the PVC's towards an other device the other pvc's running are going to terminate on the "modem"