Yes… It can be if you so desire... To me though it's a little redundant as Linux itself can manage just the same...
The ONLY thing that I've found that Ubuntu simply CANNOT do is MLPPP using more than 2 DSL lines. I used to use my Ubuntu server as the central be all and end all for the network, including running the MLPPP to make the internet work. Once I installed a THIRD line however, I began having a great deal of difficulty with getting it going on all 3 lines at the same time. So... I made the switch over to a Mikrotik router. The Mikrotik router now bonds what is currently THREE lines, but as of next week will be FOUR lines together, then shares them out as a single line across the basement to the central switch that everything connects through.
If you're not using MLPPP on more than two DSL lines though, I've found that Linux by itself seems to have everything it needs. It's just a matter of knowing how to set it all up.