If an external router is being used then bridging is preferred since some external routers have better features, WiFi, QoS and filters.
If an external router is going to use used, there would be no performance gain since having 2 routers on the line will effectively slow your data down since the 2 routers are having to process information at each point. This also causes
Double NAT and since both are trying to NAT, the connections get confused since NAT is looking for specific port and connection information. Adding another router makes this a problem.
If your are using a external router that has better features than the ISP DSL modem, then bridging the DSL modem is preferred and let the external router handle the PPPoE connection and maintain 1 NAT.
There are many variables and ways to set up networking and it's hard for DLink to always give details on what is preferred since there are many networking things to consider. DLink makes available many products to try to handle what there customers needs are. Alot of the needs are based on where it all starts, with the ISP service. Customer have to be aware of what ISP services they get and what ISP modems offer. Even Cable modems now come with built in routers that we have to configure with customers using external routers. In most cases configuration is simple. However in some cases, modems can't be bridged or the ISP lock down the modems so users can't access them at all. In some cases like this, users fail to realize that not all external routers will work for them, however maybe just adding a DLink DAP model AP to there existing modem can and is preferred to work better than using a dedicated router.
Please review this:
Bridge Mode vs Relay vs Acess Point (AP) / Routers vs Dedicated Access Points (AP)Overall, if some of the features on the ISP modem are not as good as the external router, then use the external router and let it handle the PPPoE connection. This will ensure that your data flow is working well and connections are good for Internet, gaming and such.
However if your ISP modem handles everything and maybe you only want to upgrade to better WiFi, don't get a dedicated router, get into a dedicated AP instead.
Realistically, All this is up to the users and customers.