This is how useless those cells are:
Let's take samael as an example, you got one, you want it to be 130,
So to use the elemental elementium to make this, we need to reach a point to be able to craft a samael (100) cells and leave enough (min 80) samael cells to be
able to get the rest from elemental elementium.
This point is rank 3 when you need 290 cells to rank up, so when you managed to have 290 cells, just take 100, craft and egg, this leaves you with 190 cells,
then you can use 100 elemental elementium to refill those 100, then repeat.
How do you get 290 samael cells for rank 3 you say? ok, let's say there are not samael dungeons around.....
You take your samael, use 120 elementium to get enoug to rank 1, stop, take 100 and craft an egg, do the same 1 more time so you end up with your samael
and 2 samael eggs., those 2 samaels will be needed to extract 200 samael cells from them later on.
Now, your original sammy is level 100, you got 20 cells from the previous operations, you need a 100 more for the first rank, so 100 more elementium,
you rank him up one time.
Now you need to be rank 2, so 150 cells, you have none, the use 150 elementium to turn into 150 samael cells, rank him up.
Now your sammy is rank 2, now we reached the point of needing 290 cells to rank him up. and you used 470 elementium already.
You take the 2 sammy eggs, you hatch them, feed them and extract them, now you have 200 samael cells, you need 90 more, so you use elemental elementium to
fill those 90
You reached a point where you samael is rank 2, you have 290 samael cells and 0 eggs.
What do you do now?, for your next ranks you will nedd, 290 + 370 + 740 cells so a total of 1400 cells. those 1400 - 190 you have is 1210, so you need 13 more samaels.
You have 290 cells, take 100 to craft an egg, take 100 elemental cells and do over, until you have 13 samael eggs, by doing this you would have used 470 elementium, 1390 elemental cells, to get 1390 elemental cells, you need 139 dark monsters.
You need to feed 15 samaels to 100 to achieve this, plus your original one, plus feeding 139 dark monsters, so 154 Legendaries from 0 to 100 to do this, at 2,116,220 to feed a legendary that's 325,897,880 food.
I may be off on some of those calculations, but this shows how pointless this is.
And yes, this is a rant