As sIKE suggested earlier, I think the mana conjurations have value in that they can allow you to cast things you know you need while still having enough mana to be prepared for whatever else you might need later on. For example, if you're going first and you're unsure what your opponent will do, and therefore unsure what you should play, something that gives you more mana is probably the safest play you can make, assuming you cast the next category...
Creatures. You can always use more dice as rolling dice is pretty much the only way to win, and creatures are the most flexible way to get extra dice. Creatures (sometimes other card types) with no specific abilities beyond providing dice, fat (hp/armor), and the ability to guard things, all at an efficient cost, are what I've come to call "generically good" or "vanilla." These kind of creatures are spread across level and school, and I believe a mage's ease of access to these vanilla creatures is something that really contributes to the strength of that mage, because they're almost always a good play that helps you win, assuming you play them at the appropriate stage of the game (early, middle, late). Here's a list of some by level:
Level 1 - grunts, foxes, falcons, skeletal minions, firebrand imp
Level 2 - timber wolves, butchers, slingers, gremlins, sentries, alchemists
Level 3 - the holy and dark knights/archers, bridge troll
Level 4 - grizzly
After level 4, I would say it gets less clear on which creatures are generically good to cast, but from level 5 onward, creatures either start flying, gaining enormous amounts of health, and/or ridiculous abilities like Unstoppable.
After Acid Ball came out, I think this is why spawnpoints became good; they're basically engines that make dice, and given that more dice help you win, spawnpoints reliably contribute to wins. Battleforge works in a similar, if more restrictive way.
Of course, there are cards that are not creatures that also help; Bear Strength, Rhino Hide, Regrowth, Agony, Divine Protection, various dice/armor boosting equipment, etc. They all modify dice in some way and that's why they're staples; things that modify your ability to achieve your win-con and deny your opponent's ability to achieve their win-con are strictly good in any deck. Though, you'll want to make sure you have enough offense; committing too much to defense won't actually help you destroy your opponent's mage.
Basically, I see mana flowers and crystals as a way of putting you in a position to keep spending mana on more powerful "generically good" creatures (and buffs) without being mana starved later on. The class rings should come first, however.
