It depends. Small cells are necessary to satisfy the exploding demand in mobile data, no doubt about that. The reason behind this is that data consumption is expected to rise more than 1000 fold in less than 10 years (Qualcomm's 1000x Data Challenge). There are few possible ways to satisfy this large demand, namely, more spectrum, improved spectral efficiency (bps/Hz) and spatial reuse. Obviously, more spectrum will help but the fact of the matter is that spectrum is a scarce resource that by itself will not be enough to answer the looming challenge. Certainly, using Wifi with more than 600 MHz of unlicensed band will help. Further improving the spectral efficiency will soon face a barrier after exploiting advanced schemes like OFDM and MIMO. The main contribution will have to come through spatial reuse, where the spectrum is reused over small areas. Therefore, small cells are necessary!
Now, there are two types of small cells, one for outdoors and the other for indoors (enterprise), and they are inherently different from each other especially in their business case.
Outdoor small cells will be installed on utility and light poles, side of buildings, aerial strands, etc. These small cells are carrier-specific (support only a single operator) and will likely integrate LTE and WiFi radios. Outdoor small cells face several challenges like backhaul, power, rights of way, etc. Most utility or light poles do not have fiber access and may have to rely on wireless backhaul solutions like millimeter waves. In addition, constant power availability is not always guaranteed. Also, it will take time to negotiate with real-estate owners to mount the small cells on their property. Deployments of several 10's of thousands small cells are expected during 2013-2014 with more to come during 2015.