I own the "base" model mid-2010 27" iMac. Love the machine, but looking at replacing with a MM due to a much faster computer & ease of self installing SSD, etc.
That being said, I can see one reason not to go with a base model anything.
The 2010 & up 27" iMacs are capable of utilizing 32Gb of RAM, but my "base" model i3 can only max out at 16Gb of RAM. This information was not available at the time of my purchase.
Had I purchased the next model up, I would be able to benefit from such a bonus. As we do not know what the future holds, it's nice to not be limited by saving a few bucks on the front end.