At least one of the Bestiaries is more or less required unless you want to use only NPCs. The Summon Monster spell, for instance, requires Bestiary 1 for the statistics of the creatures it summons.
Well, you can get round buying the beastiaries if you game with a live internet connection (prd access) or compile word files in advance of each session with all relevant monster characteristics.
For monster summoning spells I strongly suggest tracking down the pdf files available for printing out monster summoning cards, since they already factor in all the applicable templates and bonuses. Even if you don't plan to print a full set of cards, you can just cut & paste the 2 or 3 critters you plan to summon.
In fact, a good DM might rule that all summoning spells automatically fail unless the player has prepared the stat block in advance. So annoying to wait while they page through and add stuff up, and invariably miss something crucial.
Personnaly, I use a laptop with Combat Manager for pretty much all my encounters, you really don't need the bestiaries with that software as pretty much all of the stats blocks are already in it. It also comes with many handy links to specific monster rules and all of the spells in the stat blocks so if you forget what X spell does, you're one click away from its full description. I think the bestiaries are useful if you want to make your own encounter from scratch, but if you use modules or an adventure path, Combat Manager would be fantastic for you if you, like me, find them a bit too expensive!