On the other hand, the strongly themed lists of the warmage and beguiler often have large degrees of redundancy. So you add a new blast to the warmage, big deal. The whole reason those classes have spontaneous casting from a fairly wide list in the first place is because of the restrictive nature of their theme. As long as you stick to their core abilities, adding new spells shouldn't be too bad.
To use the warmage as an example (since I don't have PHB2), spells that are like Fireball, Lightning Bolt, and the Orbs are probably okay. Spells like the more fringe areas of their list such as Evard's or Mass Fireshield probably shouldn't be added to their list freely. Hailstones might be a good spell to add automactically, since it's just another ranged touch, Xd6 energy damage blast. Radiant Assault is a good canidate for Expanded Learning and not free access, since it adds a Will based attack which warmages otherwise lack. Similarly, Energy Spheres would also need Expanded Knowledge since it has defensive aspects AND works rather unconventional.
I'm not as familar with the Beguiler list, since much of it consists of new PHB2 spells, so I can't really provide much guidance there. PC's advice about swapping is a good idea too though.