I was hoping for a little more discussion on this subject too - I don't have cows yet but had come across reference to mini jerseys whilst looking into small breed cows.
Hopefully ohmissbrittany (hello & welcome ohmissbrittany BTW) doesn't mind us discussing on this thread. If you do bring the mini jerseys here I would be interested - though cost might be an issue alas.
My own answer to "why mini jerseys?" is probably a bit fuzzy - i read a lot of stuff on line and absorb some details but don't recall exact words/facts:
I do want to have a house cow and aside from the cute factor of a jersey they do produce a very nice rich milk. Trouble is that despite being a small breed they can produce huge amounts of milk (one for sale locally a while back was producing 5litres /day on top of feeding a calf); and correspondingly require a large amount of feed/pasture and when housed produce a large amount of muck to be dug out.
And you don't have one house cow - you tend to have 2 or more so they have company - so thats even more milk/feed/mucking out.
So a couple of mini jerseys would give you plenty of milk for a household, whilst requiring less feed/pasture,less muck to move, and be easier to handle - in theory.
And whilst there are smaller breeds such as dexter - they don't always make as good a house cow ( I think sally said something about that on the other thread) - as in not as willing to be milked by people or as willing to take on another calf as jerseys reputedly are.
Some might say: if you want less milk, get a goat - which is fair enough if all your after is dairy produce - but as a smallholder all my animals are at least part way to being pets - some very much so, so suggesting I get a goat because I would like a cow doesn't really answer.
Another reason for 'mini' jerseys is that I believe that modern cattle - even the less commercial breeds have been bred over many decades to be significantly larger than their ancestors were, as larger cows produce larger calves that are more commercially viable. So 'mini jerseys are more about breeding them back towards the size they would have been a century of two back when they might have been kept as house cows on the common by 'commoners' rather than part of a large commercial dairy herd.
If I'm talking cobblers here I won't be offended if someone chips in with the facts - like I said I've read a lot on the subject, but the internet isn't always a reliable source.