It's a bit of an obsession...
Also some of them you need to adjust for the size of the person wearing them, and as DH is 6'4" and I'm 5'6" it was always easier to have two on the go, adjusted to different sizes of person, than to be readjusting every time we switched DS from one to the other. Also DH had (gasp!) his own opinions and preferences, so sometimes he liked one sling/carrier and I liked another and it was easier to get both rather than fight about it .
The only one I ever actually abandoned was the over-one-shoulder one. Apart from that we used the hug-a-bub and Kelty interchangeably for the first 7 months or so (it was easier to breastfeed in the Kelty than the hug-a-bub, or at least I found it easier, so that was another reason for selecting one or the other on a particular day) and then the Ergo and Cwtchi after that (DH preferred the Ergo, I preferred the Cwtchi... partly, I'll be honest, because while I ordered the Ergo in navy but it turned up in terracotta and the people were so nice about it that I didn't want to make a fuss, but I don't look good in terracotta, while the Cwtchi was in lovely pretty stripes [irredeemably shallow emoticon]).
Now I've got sucked in to the Hunt For The Perfect Sling -- the Didymos and (potentially) Close Baby Carrier this time round fall into that category. Basically I really really like the hug-a-bub, and it's very comfortable, but it is a bit of a faff. So I am looking for something very similar in terms of feel and support and weight distribution but with a smaller faff factor.
Then there are all the puch-style slings that I haven't even tried but lots of people swear by, and a small part of me keeps whispering "you never know... those might be even better... why not try one, just to see..."
At least all my slings/carriers are in different colours, so I can always claim it's so that I can be a coordinated Yummy Mummy (might work better as an excuse if I looked anything like a Yummy Mummy, admittedly... or if I owned anything that coordinated with terracotta).
You can definitely do a back carry with the Ergo, and I think you can with the Cwtchi, although I don't remember from what age it's recommended (I never had the nerve to try with DS as he's such a wriggler I always thought I'd drop him while trying to get it on).
The Didymos you can definitely use in a back-carry from reasonably young (at least, have seen plenty of pictures of people carrying fairly young babies in it that way) which is one reason I've bought that in my quest for an improved hug-a-bub, as you can't do a back carry in a hug-a-bub. I don't think you can do a back carry in a Close Baby Carrier, either.