Personally,for me I'll dual box rogue and the sp in SL mainly to do the following:
1-Cut down the time for leveling/questing/rep/usual chores
2-Have fun trying some torghast push as disc/sp paired with my main rogue
3- Being able to do pvp and other stuff in the mean time (2.8xp as rogue) so I can cut down time while enjoying my own stuff (i.e xmog runs etc).
4- Able to farm proffesiosn when able to (herb,enchanting, ) to sustain myself.

Mind you, I did try 5x boxing in BFA but seeing as I am in top 140 EU guild or so I don't need to try hard for gear so no need for 5x boxing even though it might come in handy.
You should try it as see what works for you, in end maybe you'll find it super engaging but as MiRai has mentioned there are plenty of things to do as boxer other then being herb bot.

P.S
Hyped for TBC and sham turret (might try 5x sl sl lock ) : )