Ultimate Magus and Vesatile Spellcaster

[Crake]

Ok, so I've never personally decided how I would run versatile spellcaster and ultimate magus (using your spontaneous side and versatile spellcaster to spontaneously cast any spell in your spellbooks) as a DM, but it just occured to me, assuming that the trick works, in a world where scrolls are easily accessible to scribe into your spellbook why would you ever choose to max your prepared spellcasting over your spontaneous casting? There's nothing stopping you from scribing high level spells into your spellbook and casting them with your spontaneous side, despite the fact that your prepared side would never be able to do so, then just using your prepared spell slots to fuel UM metamagic?

Am I missing something here? Or are the free spells at levelup that big a deal to make maxing your prepared side such a priority?

Edit: This is under the assumption that your DM would allow you to use versatile spellcaster to spontaneously cast spells from your spellbook.

[Biffoniacus_Furiou]

You would be spending twice as many spell slots to cast every spell, and thus end up with far fewer available spells in a given day.

Furthermore, only 1st level spells should be learned via purchasing scrolls. All other spells should be learned from another wizard or organization at 50 gp per spell level (http://www.d20srd.org/srd/magicOverview/arcaneSpells.htm#arcaneMagicalWritings), or perhaps even free if you have a spell of equal level to trade for it.

[Crake]

You would be spending twice as many spell slots to cast every spell, and thus end up with far fewer available spells in a given day.

Yeah, I get that, although it's partially made up by the fact that using versatile spellcaster, you could access spells of 1 level higher than you could normally cast (which, when you consider that spontaneous casters are left behind a level, plus at least 1 level of a prepared class to qualify for UM, puts you at normal spell level access for a spontaneous caster) and the fact that spontaneous casters have more spells per day (at least than a non-focused specialist) wouldn't it be at least a close trade, gaining the ability to spontaneously cast from a much larger set of spells?

[Biffoniacus_Furiou]

A Beguiler gets only one more spell per spell level than a specialist Wizard. Furthermore, at the odd numbered levels the Wizard gains two additional spell slots of the next level higher, plus bonus slots for a high Int, while the Beguiler still has zero. You're better off being a Wizard who's one level behind, and able to spontaneously cast 1st-5th level Wizard spells when needed, than a Beguiler who's a level behind with access to Wizard spells at double cost.

If you want to make a Beguiler who uses this trick, don't take any Wizard levels at all. Just get the feat Magical Training (PGtF) to get a spellbook which you've already scribed several spells into, so per Rules Compendium you can add more spells to it just like a Wizard does.

[Crake]

If you want to make a Beguiler who uses this trick, don't take any Wizard levels at all. Just get the feat Magical Training (PGtF) to get a spellbook which you've already scribed several spells into, so per Rules Compendium you can add more spells to it just like a Wizard does.

I guess that eliminates the middle man entirely, didn't realise a feat like that existed, but it would certainly make it all quite a bit simpler.