This whole discussion gets to the core of role-playing. The books present a set of rules that are used to outline the mechanics of the game. Although I have only been playing 3.5 for a little while, when I started, I read each of the Core Books (MM included). The reason is pretty simple; as a player I want to know what the rules for my character are, and if I chose to DM, I wanted to be sure that I was familiar with the critters available to throw at my players.
What this translates into at the game table is that I have a fairly solid understanding of the rules that govern a myriad of unusual situations, that my character should not have. It is up to me as a player to choose if I want to use it for metagaming or to roleplay. I have a tumble score of 16, so it means I am pretty darn good at it. Being fairly well trained in that skill it is reasonable that my character would know that trying to tumble through an enemies square is really quite hard (DC 25). Knowing that if I try to tumble past that critter will generate extra attacks of opportunity because it has a special quality I read about in the MM is pretty bogus.
The onus is on the DM to create a sense of mystery by saying things like "you see a ugly green humanoid that has a sickly pallor, as it shuffles closer, you can smell rotting flesh, and and it is missing small patches of skin" to it instead of saying "you see a Zombie Goblin". Of course, this can be set aside to a certain degree if you are on your tenth zombie goblin encounter in the Temple of Zombie Goblins. Should we get to the basement of the Temple and encounter a well rotted zombie with worms crawling on it, if I shout out "Oh no! A spawn of Kyuss!" I have taken a wild step over the line of meta-gaming.
When I want my character to have knowledge that I as a player have, it is my responsibility to work with the DM to create a reasonable scenario where I can have that knowledge and act on it when playing my character. After the above encounter with said spawn where MeatShield the Barbarian was infected by a spawn worm causing us to retreat, we visit the local clergy. While he heals up MeatShield, the priest explains that they have had many adventurers come by lately with similar problems and that there is a nasty zombie in the temple, and provides some insight about how to destroy it and defend ourselves while doing so.
Ultimately how the game is played is up to the players; you should be playing the game to have fun. If you, your players, or your DM are not having fun, that is an issue that needs to be fixed or your campaign is going to fall apart. I would suspect though, that if fun at the game table equates with removing access to the rules, or a portion of them, from players, then I think it is time to re-evaluate how the group or people in the group are playing the game.
I do agree that looking up monsters mid-combat is pretty lame, even if you are going to cast a summoning spell; if you are familiar enough with the spell and options, then it is reasonable, otherwise, you need to spend time between sessions familiarizing yourself with the options available to your character.
