If you plan to fish to higher than 50, Lu Shangs isn't really optional. But on the same note, you really don't even need it til 50-60. The other rods either will not tire the fish fast enough, will swing too much, or will break. You'll also lose a ton of lures, and since most people fish for gil, that sucks. So just get it.
I just recently got mine after 8 months of on/off fishing. The best way to do it so you don't go insane is to vary your fishing. One day, I'd head to Knightswell in E. Ronfaure and fish nothing but carp. The next day, I'd take my Halcyon and go to Bubu Peninsula and fish shall shells (which cap at 53, and I used to go from 30-53) for both skill and money. Sell the shall shells and buy carp. Another day, I'd fish in Windy and pull up moat and gold carp. Moats for the quest and Gold for skill. It actually goes pretty fast and I could have done it much quicker, but I also have an endgame shell and other stuff to do.
If you have tons of gil, you could buy the rod but I don't recommend it. If you're like 10 fishing you can easily skill up and do the quest at the same time as I outlined above and save yourself 2 million.