In my case, the reason was primarily cost. I wanted to get into BBQ after moving to the US, and since I'm British, I just thought that BBQ meant direct heat grilling over charcoal. I actually thought that when my father got a gas grill back home, and started using it for indirect heat cooking that he was cheating and that wasn't BBQ at all. Although he wasn't cooking low and slow, and didn't have any smoke wood at all... I didn't realise that indirect cooking was an important part of BBQ until I started researching what to buy.
During the research, I stumbled upon this amazing site, and really got interested in 'low and slow' cooking. I couldn't really justify the expense of a real WSM because I had never done low and slow BBQ cooking before at all. The size of a real WSM was a little off putting as well (I'd like to store the cooking in my garage, and there is only the two of us eating from it - my son is only 14 months old). A mini WSM was a very cheap introduction to real American BBQ, and so far I'm really enjoying it.
I might actually consider buying a real WSM because I found the small 14" grills a bit small for some larger cuts of meat. I used the second grill level for the first time last weekend though, so now that I've 'commissioned' that, I do have some more space to cook on.
Hope that helps.