Yes, you heard me right - a 2nd lv cleric spell negates a 7th lv spell completely. That is a fairly common problem in 3e - abilities are either broken or useless depending on what sort of countermeasures are in place, few in-betweens.
If I was DM, and I knew the party cleric prayed for this spell often, I'd augment my balors with quickened spell-like ability: blasphemy, bombard the cleric with greater dispels at range, then hit the party with a quickened blasphemy immediately afterward. During the following round, I'd use blasphemy/quickened blasphemy while the other balor (the one that was summoned, not gated) wades into melee, using the coup de grace where necessary.
After all, the demons should converge on anyone who appears capable of using banishment in any way. Holy symbols, an overall pious appearance, blunt weapons in conjunction with heavy armor, divine spellcasting (such as silence), etc. are all giveaways.
Also, keep in mind that a silence spell negates any verbal coordination between PCs, whereas the demons can speak to each other telepathically if within the radius of the silence. That's assuming the players role-play properly and avoid blurting things out that the PCs shouldn't be able to hear.

That's something my players would do, and they'd feign ignorance in the process. Very annoying.