No point of EG/HT belt skills at sametime
But it is very important in sPvP. In WvW are many things useless.
stacking regen duration can be helpful. plus healing mist is a stunbreaker and regenerating mist is a water field.
I find it very handy, in wvw where there is oftenly a necro or mesmer getting around screwing with my boons it’s handy to have multipul ways to reapply regeneration, if you are spamming them together to maximise the duration then your doing it wrong
and as ellesee said 1 breaks stuns and another makes a water field which is rather easy to quick hit a combo skill to get a 1k+ heal.