For me, wala. Kung meron ka mang alt, depende narin siguro sa kung ano gusto mong gawin with it. Kung gagamitin mo ba ito as back-up or for cheating multiple bounty campaigns.
Mostly talaga sa mga nakikita ko na may mga alts, they are using it to cheat on bounty campaigns. Marami diyan from a mere two, to a hundreds ang mga alts nila.
BTW, yung pagdami ng mga alts for means of cheating a campaign is one of the reasons why merit system is introduced.

None. This way I can focus on my one and only account.
Bottomline is, hindi masama ang pagkakaroon ng alternate account/s if you are only planning on using it as a back-up account and not for cheating.