Most people I know prefer DNC Merited Haste Samba for the additional TP and possibly superior DPS. Enspells are really only good, not great, when you are against something with incredibly high defense and low magic defense. When your THF hits Jailor of Fortitude or PLD mobs for 1~10 damage, it's nice if they can hit for 27 damage on hits with Enspell 1s.

But hey, if they want to make it so Enspells and Sambas can stack, then I would be happy to worry about if my party has Enspells up. Otherwise I'm undermining my DNC.
Obviously another 10% JA haste would be superior to anything enspells could do without a very specific situation. DNC is also kinda of a rare job, I don't see many in groups and I've rarely partied with one. I was assuming no DNC present. Of course that opens up the possibility of RDM/DNC giving 5% haste samba, depending on situation could be very useful.