Right, that makes sense, certainly. My point above was mostly related to general-purpose internet traffic, where special-purpose use cases like yours are rarer. Also, if you could just install sch_cake in host fairness mode on those switches, your IP cameras would be fine unless you had so many clients transmitting simultaneously that you would run out of bandwidth entirely (e.g., if you have a 40Mbps link and each video stream is 1 Mbps, you'd be fine with host fairness until you exceed 40 simultaneous hosts transmitting).
Also, I'm not denying that it is possible to optimise traffic some with finely tuned diffserv markings. I'm just saying that it's not worth the effort (to me). I.e., just turning on sch_cake and configuring the bandwidth gets you 90% of the benefit compared to an unmanaged link, and spending hours tweaking things to get those few extra % towards some theoretical optimum is not worth it 
However, using diffserv-based prioritisation to work around devices in the network that lack the smarts to do better (as you're doing) is certainly viable. To me, that is the justification for diffserv in general, although thankfully I don't have any bottlenecks in my own network that needs this kind of tuning