From a home perspective, the best way to think about a (full depth four post) rack is as an extremely high density shelving for specific types of things. You won't make any gains with one or two machines, or several SBCs in small cases, that you couldn't have made with similar non-rack shelves. Having a few pieces of gear with rack ears not in a rack will forever call to you though. In general I'd either say commit to it (42U and understand you'll be paying a premium for shelves/gear/accessories), or regular shelving will suit you fine.