They'd effectively be creating more work for themselves and cutting the over all benefit by reducing the maximum amount of coins. A 10 x 1MB hash won't be able to compete with a 10MB hash, simply because the 10MB is much faster... it's like trying to compare a ferrari to 5 ford transit vans. So if you reduce the maximum range then everyone simply gets a smaller chunk. The key here is, everyone gets a chunk it doesn't matter the size and this means that everyone can take part so long as they have contributed to the mining process.

Mathematically (theoretically at least from my back-of-envelope sums) those who are mining at full pelt 'should' feel better off by mining as much as they can, than they would by splitting into more parts and consuming more energy in order to do so. The bigger the range the better the reward for everyone, so it's in everyone's interest to mine as hard as they can and not just the largest trying to price everyone out by getting the biggest machine because it defeats the entire point of why they're doing it in the first place.

I don't doubt that there will be some who split their computers down into different identities, this will slow down their speed of processing and enable smaller miners to have a better chance as well especially later on. So it's kind of self defeating.

Now...there is another aspect that needs to be answered because mining will eventually become too difficult (I'm hoping a long way in the future) and that's how the transactions are handled/crunched through and the reward mechanism tied into that.

Edit.

After re-reading your statement. If they set up 2 computers of both 10MB hash rates...then yes, they would technically gain a larger portion. However my calculation is shared between the number of users, so the more users the more the decrease in the kickback. With that said, if they're doing more mining why shouldn't they be rewarded? They'd find more reward  in the 89% mining reward rather than the 10% kickback.