Against undead Beguiler's spell list is reduced to Figments, Glammers, and Shadow as illusion subschools.
The entire Enchantment school is just out against undead.
Looking at the Beguiler's spell list there just is not that many High Level Figments, Glammers, or Shadow sub school spells on their spell list. Shadow is the key subschool here, with spells like Shadow Conjuration, Shades, and Shadow Evocation being chosen as the advanced learning options.
I guess a Beguiler could get by like that, playing like a Wizard with a LA penalty, but with no benefit. Just seems a shame to be reduced to being a mere conjurer of near real things when against living oponents a Beguiler can act like Obi -Wan Kenobi, on the good side, telling people to "go home and rethink their lives",
or "Where not worth you trouble" to , representing the dark side " kneel before Zod" moments.
The problem is what campaign is not going to feature Undead, or other Immune to Mind Affecting things creatures (eg god)are you going to find. Pray for the campaign against Demon Lords I guess.