I have a wireless router and two access points...so, no, it isn't replace 1 with 2+. It would be replacing 3 with probably 4. My house isn't huge, but it is two floors and long, plus lots of outdoor space (40,482sq-ft of it).
Clients roaming isn't an issue for me, what is an issue is that Apple has done a crappy job of designing their network stack and has only been making it worse over time. My iPhone roams fairly well, but sometimes if something is "running" and it roams, it'll "freeze up" for awhile. Example, checking email, if I walk across the house as it is trying to check my email, the client will sometimes just sit there for 30 seconds thinking about it before it'll finally pull down any new emails. Or I can force close the email client, pull it back up and then it'll pull the new emails in a second or two.
Facetime will tend to either drop connection quality to voice only for 15-30 seconds if you roam between the access points, or occasionally it'll just disconnect. Which is actually slightly better behavior than what it used to do, but the email thing is new (Netflix BTW roams gracefully on my iPhone, it is only 1st party iOS apps that seem to have the roaming issue). Same thing with Safari loading a page.
Windows does not do this at all. Android doesn't seem to do this.
iOS (other than Facetime) was actually a lot better about this under iOS5, 6 and 7. In fact my wife's iPad 2 is still running iOS 7 and it works pretty well roaming around (again, exception Facetime, which has always been an issue). iOS8 on the other hand, crappy.
I've seen/heard tell of added in issues with OSX over the releases as well (such as OSX10.6+ removing the ability to force the adapter to connect to one particular band on a dual band unified SSID). The more Apple tinkers, the worse they seem to make networking.
If the iDevice under iOS8 isn't actively trying to pull down information on the network when it roams, all is good. But first party apps flip feces a lot of times if they are and you roam. ZHO very well may fix that. Again, Windows has no issues with it. The worst I'll see is a network transfer bottom out for 2-3s before it jumps back up to full speed. Skype roams gracefully with at worst a second or two pause in the video stream.
Sometimes Apple really needs to fix. So, ZHO isn't mandatory for me, but it sure would be nice. Since the hardware and some other Ubiquities other products support it, I'd wait on ZHO to be a real working feature before considering the new 11ac APs from them. The price is nice, but I also already have pretty decent wireless speed and coverage around my house (all 11ac, except outside which is only 11n 2.4/5GHz). The only things they would add is making it a little easier to deploy a dense 5GHz WLAN because of the form factor and the possibility of ZHO...which doesn't exist right now.
