In short: versatility.
The reason that prepared spellcasters such as the Cleric or the Wizard are generally seen as Tier 1 is because of the sheer versatility of the spells they can cast: no matter the situation, they have a spell that's tailor-made for it.
No all spellcasters have the exact right spell, though, and therefore part of the optimization game for a spellcaster is finding ways to access spells that they would not normally have access to; such as spells from another spell list:
- either to obtain superb/broken spells from another list,
- or to obtain spells that cover blind spots in one's list.
A caster's power is pretty much tied to the variety of spells it can tap into, so anything expanding this spell list is worth a look.
Anyspell, and Anyspell, Greater, give access to spells from another spell list. There is indeed a limitation of 1/day each, and a written copy of the spell is necessary1, however in exchange...
When many class features allow you to pick one spell off another spell list when leveling up, or give you (gasp) access to one domain, Anyspell has no such limitation: every use allows you to pick a new spell. Late choice means choosing when you have information to inform the choice, rather than picking a spell or domain several levels before and hope they prove useful.
Furthermore, Anyspell gives access to the most versatile spell list in the game. For comparison, checking a database:
- Level 1: Cleric 149 spells, Wizard 297 spells (+99%)
- Level 2: Cleric 211 spells, Wizard 434 spells (+106%)
And Anyspell, Greater only builds on this to give access to 3rd, 4th and 5th level spells -- 399, 355 and 313 spells respectively.
So, Anyspell gives you, within 15 minutes, access to any one of 731 spells and Anyspell, Greater gives you, within 15 minutes, access to any one of 1067 spells.
1 A Cleric intent on using Anyspell should find ways to procure a spellbook, with the right spells. Luckily, low-level spells are commensurately cheap; regardless of their utility.
But once per day?
Yes, once per day is quite a limitation. It's a limitation for any prepared spellcaster, though, and therefore there are often ways to work around it both in official and 3rd party material.
For example, a Pearl of Power of 3rd level (9,000 gp) and 6th level (36,000 gp) will allow to recall the spell (1/day) if necessary. I would expect prepared spellcasters to have such pearls at hand, as they are so useful.
Another example would be Miser with Magic (Kingdoms of Kalamar's Player's Guide, p. 88), which allows getting up to Int1 spell levels/day back with a simple Spellcraft DC.
However, keep reading, there's better ways for Anyspell...
1 If you use this Adaptation PDF, then the feat requires an Arcane Caster Level 7, and uses the spellcasting attribute rather than Int.
But it means taking the Spell domain!
Well, the Spell domain is a hardly a slouch: Anyspell (3), Break Enchantment (5), Anyspell, Greater (6), Limited Wish (7), Antimagic Field (8) and Mordenkainen's Disjunction (9). That's a very solid combination.
And... as usual for D&D, there are other ways to access the spell. The very broken Initiate of Mystra feat (Player's Guide to Faerun) also grants access to the spell...
Initiate of Mystra
There are only 2 requirements for this feat: Cleric level 3rd, and patron deity Mystra. Easy, peasy, for any Cleric of Mystra.
The basic effect of the feat is to give the Cleric of Mystra access to 6 new spells: Spell Shield (2), Anyspell (3), Spell Phylactery (5), Anyspell, Greater (6), Spellmantle (6) and Holy Star (7).
In doing so, it also lifts the domain slot restriction of Anyspell and Anyspell, Greater, allowing them to be cast as regular Cleric spells: that is, from any spell slot of appropriate level.
And of course, the most broken aspect of the feat, is that it allows a Cleric to cast in a Dead Magic zone or an Antimagic Field, with a relatively simple Caster Level check.
I concur, those spells are broken good. Especially when their only limitation (1/day) can relatively easily be worked around.