Thank you for answering my previous question regarding the name Mackenzie. I am fairly sure we will go ahead and use it if we have a girl. Now, on to the middle name.
And now, a quiz- if I told you I was born in 1980, I bet you could figure out my middle name in three guesses or less. Around 95% of my friends have either Elizabeth, Marie, or Ann as a middle name (mine is Marie). Do you see where I'm going with this? I love the name Grace. It's a great middle name because it goes with everything. It's feminine and is perfect to offset the spunkier Mackenzie first name. Mackenzie Grace. Perfect, right? Except I've lost count of the number of baby girls I've come across in the past year or two with that middle name. It seems to be the go-to middle name of the 2010s. In 30 years, will little Mackenzie be saying "I was born in 2012, so of course my middle name is Grace"?
I wish so much that the Social Security Administration would let us snoop around in the middle names as well as in the first names. I'm sure you're right that Grace (along with Rose and Elizabeth) is one of the main middle names of this current generation of baby girls---but I'd love to know to what EXTENT we're right. I suspect that although Grace and Rose are the Ann and Marie of today, they're still not used as much as Ann and Marie were.
I think the attitude toward middle names has changed a little. I could be completely wrong about this ("feeling like" something is a certain way is what leads people to say that their child's name got popular AFTER they used it, even when that's not the case), but it seems like middle names didn't used to get quite so much attention. It seems like they used to be more of a "tie the first and last names together" name from the "sounds nice" category---whereas now there's more careful agitation put into the choice. There were PLENTY of people agitating over choosing meaningful interesting middle names for their kids in previous generations, and there are PLENTY of people choosing Rose and Grace as this generation's sounds-nice bridge names---but I'll bet if we had statistics (OH HOW I WISH WE DID), we'd see middle names doing what first names have been doing: spreading out from a smaller selection to a larger one. And if we could do a massive survey, I'll bet the answer to "How did you choose the middle name?" would be more heavily slanted in previous generations toward "I dunno, it just sounded nice" than it is today.
All this is to say that although Grace is certainly a popular middle name for baby girls right now, I'll bet today's popular middle names are used for a much smaller percentage of baby girls than the previous batches of popular middle names were---just as the 2010 #1 name Isabella is used for a much smaller percentage of baby girls than the 1970s #1 name Jennifer was, or than the 1950s #1 name Linda was, or than the 1930s #1 name Mary was.
I do think that if I were choosing a common/current name as a child's first name, I'd try to avoid a common/current middle name. There can be some fun in a shared name: my son William frequently has the same two other Williams in his classes at school, and they say "Hi, William!" "Hi, William!" to each other and crack up every time, and they'll refer to themselves as "the Williams" and so forth. But for me, the fun dissipates if I picture saying to one of the other William mothers, "Oh, what's his middle name?" and hearing it's the same as my William's middle name. It goes from the "We have such great taste, don't we?" feeling of showing up at the party with the same dress, to the "I guess we both saw the same mannequin" feeling of showing up also in the same shoes and jewelry.
If your grandmother's name were Grace and you were asking if you could go ahead and use it, I would be all for it. I'd remind you that even if there are lots of Mackenzie Graces out there, it's not like she'll be running into them every single day. I'd emphasize that I think it's wise to let the love/meaning of the name trump popularity concerns. I'd mention that one-syllable bridge names are few and far between, and that they really do make a name sound great. And I will STILL emphasize those things: if you love it, if you WANT it, then USE it. But if you just like the sound of it, I suggest finding another, less common name you like the sound of.