Szeth is a Radiant now. The decision to swear his ideal to Dalinar seems like the first time he actually made a choice for himself rather than be someone else's play thing.
Also, as a bondsmith, Dalinar couldn't just ignore Szeth's oath. In accepting Szeth, Dalinar made another step toward uniting them.
I'm not sure when exactly but for some time I was expecting Szeth joining the good guys. I agree that it'd be nice to have more details and see more interactions between him and the rest of radiants but I guess it'll be delved into in the next book.
As to Moash's Shardblade and Plate, no one really asked when Kaladin decided to award those to Moash. Why should they question Kaladin's decision now? At the time, Kaladin himself wasn't sure what to do about Elhokar. Seeing Moash killing Elhokar send him to a dark place.
Please correct me if I made any errors. I don't know how you lot are able to keep all those details, plots and subplots in your minds