Is it really pointless to have all die facets?
Or do they stack?
Is it more economical to have only one die facet inserted into your gear and the rest lvl?

no

Only need 1 rest lev up

just looks better when all are die

They don't stack because it's the same level. If you had a HoJ runeword for the chance to cast meteor on death and a die facet in your helm which is a different level meteor then it casts two different meteors on death.

If you synergise meteor fully on a sorc and pop a hoj on her you can deal like 80k damage or something on death. It has its uses.

They don't stack so you can go for the cheap way if you want to