Typically the SLI motherboards have the broadest feature set of the enthusiest motherboards today. For example, compare DFI Ultra-D to SLI-DR or SLI-DR Expert. Theres a lot of added features besides SLI support. Most companies do this, it seems.
I run SLI myself, but you need to look at individual motherboards and not just take my word for it, compare an SLI board from a company vs another and see what fits your desired feature sets. Look at things like hard drive RAID controllers, ethernet controlers, USB ports, better sound chips, possibly cooling designs (like heatpipe technology on the newest Asus SLI motherboards). These types of things may be desirable to you - and they have nothing to do with SLI.