It may not be "wrong", but certainly is unnecessary for the application that I am presenting. We could simply be talking about two different scenarios here.
In the specific application of a DSL line, you might need such a filter or a filter/splitter. That's a very specific application and use and has nothing to do with ethernet as it does with the fact that it is DSL.
On the other hand, if I am simply plugging into an ethernet jack with an analog device and using that run all the way back to a block, it's a straight copper run. More importantly, it's a straight copper run on a cable that far exceeds the Cat3 standard for such devices.
Perhaps if you can explain the purpose of this filter you keep referring to, it might help me understand where our disconnect is?
I honestly don't know how much experience you have in an enterprise data center, but even a quick google search brought up the following:
"It is pretty common to see UTP Cat-5, 6 or 6a cable being used for phone lines in newer construction or installations, the cost of the cable is not that different and it just makes it easier for the installer to just have to run one type of cable for both phone and data. You can even use RJ-45 wall jacks for phones because RJ-11 plugs will fit, though you can only access up to two pairs or 4 wires in the RJ-45 wall jack.
You can see this in many office buildings, and if they use RJ-45 jacks for both data and POTS phone the jacks should be marked and are usually a different color. It the business uses ISDN or VOIP then the wiring needs are different, but can be usually be supported on Cat-5, 6, 6a cable and RJ-45 wall jacks"