The payoff is immediate the next round, when you can "afford" the more expensive items (that I wish to cast) sooner rather than struggling to save mana between two rounds to enable me to cast a costly spell. Yes, there is a small upfront sacrifice to tempo when starting, but when you are out in rounds 2-5 you will not be anywhere as mana starved as you would of been channeling just 9 mana a round.
I think you need to compare spending the 10 mana to cast the two Flowers or crystals with not casting anything at all, instead. The question is... "is it worth it?"
If you cast these two, you spend 10 mana (8-9 for Crystals with Arcane Ring). Instead, you could choose to not spend that mana, and have it immediately available to build those five 11 mana creatures that you were desiring. It looks like this (excluding other misc. spells cast):
Start with 19 mana (presumed)
Round 1 = Cast two flowers -10 mana, +2 channeling; End mana = 9
Round 2 = Cast 11 mana creature - 9 mana (net); End mana = 9
Round 3 = Cast 11 mana creature - 9 mana (net); End mana = 9
Your total mana available to cast two additional quick spells in rounds 2 and 3 is 9.
Compared with:
Start with 19 mana (presumed)
Round 1 = Cast 11 mana creature - 11 mana; End mana = 8
Round 2 = Cast 11 mana creature - 11 mana; End mana = 6
Round 3 = Cast nothing; End mana = 15
You get
four additional quick spells costing
15 mana or less in these three rounds also, compared with two additional quick spells costing only 9 mana or less, all while getting your creatures out more quickly.
It seems better in the short term to not cast the flowers, but to bring out the other spells that you desire instead. You don't pay the opportunity cost of the flowers and lose mana early on that way, and you perhaps do not lose tempo doing this.
In the long run, if the flowers or crystals survive, you will have built up enough mana to overcome the short-term mana shortfall.
I'm just playing Devil's Advocate here, though. I prefer to have one or two mana generators out on the board and/or having my mage harmonized and/or wearing an amulet for additional mana, rather than not having any of these things.