They Provide, food drink, warmth, and make good companions. 

The down fall is they can be incredibly expensive.  Shoeing, feeding and sheltering these animals requires a considerable investment.

 It is best to only have one (If you try to maintain a herd it could be detrimental to more thing than you can list) 

However when shopping for one, if you choose the correct one, it will last you your lifetime.

There is nothing like the warm glow of a burning bridge.
Hung like Einstein, Smart as a Donkey

My Border Collie is the MOST useful creature on our farm. We can't eat him, but if the chickens need put in or the turkeys brought back from the neighbors, all I have to do is say, "Hey, Bandit!" and point. He knows what to do. He's my big baby, our future stud dog, and the best farm dog I could imagine!

As for Harlequin rabbits, I've raised them for about four years. They have fantastic personalities and are beautiful, which puts them in a great market for pet stores, 4-H and FFA kids, etc. Also, I show them, and I can easily pick up a poorly marked doe for $5, or even free, and breed her and she can produce some beautiful babies. The best part? Even though most have very little muscle (because they are bred almost entirely for markings) they are big enough that if you own a nicely muscled strain, they dress out GREAT! Don't know what they look like? I recommend googling it :) or if you check out rightpet.com, their Harlequin photo was one of mine that I let them use!

In long-range planning for the farm, we wanted to put in a vineyard but didn't want the mechanical/chemical input required for a good crop of grapes.  We found the answer to be Old English Babydoll sheep.  At 2' tall they won't be able to graze on the vines or fruit at 5' but will still provide fertilizer and herbacide coverage...and when the flock is big enough, either meat or sellable stock.  Plus, they have great wool which we spin and knit.

Cattle: We started out by accident with a pair of Holstein steers.  A great learning curve because they became spoiled brats with no respect for fences of any kind.  I had NO mixed feelings when it came time to take to them to the locker for processing.  And they do taste good!  We gave a lot of thought to what we wanted on a long-term basis and considered Dexters until I met some and, gentle though they were, I was intimidated by the horns.  Then I met a Lessor Jersey!  We have one now and, while not a small investment, she is, as has been said before, a big puppy.  Just need to get her AI'd now for a calf and milk production as we too, would like milk and cheese fresh from OUR source and to develope a small herd.  From what I've read on various posts here, I would consider standard Jersey steers for meat.  But would eventually like to include some small Hereford's for meat as I remember, from my younger years, their personalities were sweet and gentle and their meat production was great.

Cat's: we had one when we bought the farm and a lot of rats and mice in the chicken house.  "Harry", named because it was a long-haired ferral cat should have been "Harriet" because she produced 2 kittens 3 years ago  which produced 4 kittens last year and 8 this year.   Needless to say, we have NO rodents in the chicken house or the barn!

Dogs: we have one, a cross between Golden Retriever, Chocolate Lab and Collie.  She has the best of all their personalities and is the best friend and guardian of our property I could wish for.

I'm steering away from hogs because I'm under the impression they need fencing (to prevent rooting under) that I'm not willilng to provide.  Same with goats (going over fencing).  The previous owners had 500 goats here at one time but I'm not so inclined. 

I would love to have  horse again as I did in my youth.  But our plan is now, 'if it can pay for itself in some way, it can stay.  If not, it's not needed'.  A horse won't pay it's way at this point.  Don't know if this helps or not but you're welcome to it.