I vote against that. That's the 3e solution all over again, with the MM having base monsters that you can add templates and/or classes to.
The main problem with your idea is that it takes a lot of setup time. If I want to have a fight with an orc raiding party consisting of three melee guys (basic orcs), two axethrowers, and a boss, I would have to write up the leader and axethrowers myself. With the 4e MM, I just plonk in two Orc Berserkers, three Orc Raiders, and an Orc Eye of Gruumsh.
A second problem is that it doesn't take unexpected synergies into account, which a more holistic monster design system might. I'll give you an example of a monster that broke the system in 3.5e.
I started with a mind flayer (XPH version, which acts as a 9th level psion instead of just having a few spell-like abilities) and added one level of monk. This single monk level gave me a humongous AC increase (because mind flayers have very high Wis), Improved Grapple (+4 to Grapple checks, which is nice because the Mind Flayer's grapple check sucks for something that's supposed to be its signature attack), and one extra HD. The extra HD took it up to 9, which meant I got another feat. I spent this on Expanded Knowledge: Grip of Iron (a psionic power that gives a bonus on Grapple checks for a couple of rounds).
For a single point of increase in CR, I got something like +5 to AC, +2 to all saves, and +12 to grapple checks (as well as some hp, and some other incidentals like "elite" stats). This is an extreme example, but it shows how a system where you build monsters out of building blocks can break down.
Well I don't mean a direct PC type creation system, but one that provides for EACH monster to be able to be one of the monster roles, while adjusting +/-5 levels. you would really only need to buy the book, and make each monster once for later use.
say our kobold was the list of stats. You turn to the page that tells you how to make this base monster into the other roles. say all monsters are considered solo for the base. This stuff tells you how to make your other things like brute, minion, etc. you go ahead and write them up and when you have all 5 monster roles covered, you are done. You just keep them in a notebook, or on index cards, or whatever. This means each monster can be any role, and the books themselves could have more monsters in them by having less mosnters in them. Or the MM could have less monsters in them but give you the chance to have more than the basic monster for a lower price.
I am not talking about taking a mind-flayer and giving it some vampire template then add an ethereal template and that stuff. Just taking a single entry for kobold written up, and then a system for making that kobold and any other monster from what exists in the brute, striker, etc monster roles.
So the book contains:
kobold
[stats]
monster role system
from this you can create:
kobold -solo
kobold -brute
kobold -striker
Saves paper, printing costs, book size/weight, consumer costs. You have a one time task as a DM to convert the monster stats into the roles. You pay less, but have only a little bit of extra work to adjust the monster to be for any roles. Then adjust for level after making the monster fit a role.
It is work that only needs to be done once.
The encounter budget system can easily support this as well should the level range for monsters in an encounter to keep things balanced for difficulty.
So It isn't like you would need to create your kobold brute 6 times because you need to use it in 6 different encounters. Just like you don't buy 6 copies of the MM because you need 6 kobolds in a single encounter. You reuse the set of stats you made the last time.
Any weaponry would be limited like your axe to the role of the monster and given:
Kobold: [stats]
Brute: uses axe for weapon
striker: uses sword and board
solo: uses these powers.
The extra stats entries would be just a few lines and you do the math yourself for the new HP, AC based on the system to save space in the book.
THEN if people wanted prewritten stat blocks WotC or a 3pp could sell monster cards with the fully stated monsters for quick use. (if the GSL allows for making things like cards for monster stats, powers, etc for GSL made material.)
I really don't see a problem and would love a small book of many monsters that just tells me how to adjust for the other roles, and then might later offer card sized monsters of those roles or no cards at all. cheaper and greener is good for me. I will do that little extra work to turn one monster into 5 by adding the roles, and when needed those 5 into 55 to have the. adjusted for levels for when they might be needed.
Think how many monsters are currently in the MM. Say it is 100 individual races. That would be 500 monsters when each on is made into the various monster roles, and a total of 5500 monsters in the MM when adjusted for levels.
I can't recall the last time I bought a MM for ANY price that had 5000+ monsters in it!