I think most people are not thinking about Spawnpoints correctly. The best one is the Lair, and I will use it as an example. It is essentially 2 Mana Crystals + a creature with 4 (or 5) dice attack and near-indestructibility for 15 mana. The crystals are each worth 5, and the creature is worth at least 9. And, these are super mana crystals because they have more armor and life than normal.
Whoa, you say, what is this creature? It's your Beastmaster! Because his action is no longer being used up summoning Level 2+ creatures, he can now move around engaging in combat and guarding things. I feel like a lot of people say summoning a Spawnpoint is a tempo negative play, but it can actually be a huge tempo positive play in a case like this because the Beastmaster himself is about as good as most level 3 summons when it comes to combat. (He's worth 5 dice of damage if he's battling in the same space as his pet.)
It feels like you can't do much better than getting ~ a level 3 summon and a double Mana Crystal that becomes "profitable" within 3 turns!
The value of each spawn point varies with the mage and the Spawnpoint:
Warlock/Pentagram - The warlock has melee skill, and his spawnpoint combos pretty well with his desire to summon a few big guys and be aggressive. However, the spawnpoint is pretty vulnerable, so it's pretty dependent on how much access to Ethereal damage your opponent has. Oddly, the more aggressive your opponent, the better it gets because you are both i) more likely to max out the mana per turn and ii) your blood pet (or whatever your first summon is) will be more likely to be in range of a target sooner.
Wizard/Gate to Voltari - Gate is as good or better than Lair: it is durable, should always produce 2 mana a turn, and will often produce 3. The Wizard wants targets for Harmonize, and the gate provides a good one. Having a Gate either opens a Wizard to keep investing in infrastructure, to cast lots of smaller spells/multi-summon, or to divert his action to meditating to gain vast mana advantage.
Priestess/Temple of Asyra - Probably the worst combination. The temple is a little fragile, never produces a great amount of mana, and the Priesstess' main action is one of the less valuable. Unless the game is looking like you can take the time to Harmonize the temple, I could see the argument for just skipping it since it doesn't reliably offer the ability to summon every turn. Without freeing your Wizard's action, I agree the economy on these cards isn't hot.
Druid/(insert Tree) - For the Druid the trees are mandatory since they come with the built in +1 Channel. They also feed into her core strategy of meditating in the corner while summoning via vines elsewhere.
Necromancer/Graveyard/Librum Mortus - Graveyard is very powerful in that it probably generates the most mana for its cost of any Spawnpoint, but not usually in the first few turns before there is much carnage. Librum Mortus is cool in that it summons where you are, which could be convenient if you're getting aggressive. Either way, getting an additional attacker, or being able to summon 2 creatures a turn (fully leveraging Death Ring) seems like a great thing to do.
(I haven't played FM vs. WL so can't comment on those.)
It should also be noted that for mages being aggressive, the ability to throw a Spawnpoint in FC on Turn 1 represents a gain on the order of a Force Push in terms of getting your summons to the enemy gate. A Fast summon spawning there is in the opponent's business on Turn 3.
