You're exactly right. The show needs to stand on its own. You wouldn't want Iron Man to swoop in and save the day in a Captain America movie, or Thor to beat the bad guy in an Iron Man movie. The focus -- and the victory -- belongs to whoever has their name in the title. For example, when Sif showed up, the SHIELD team didn't become supporting players in her story -- she was a supporting player in theirs. Her presence and her conflict with Lorelei served the arcs of the AoS characters -- Coulson's search for answers about the alien, Ward and May's relationship issues coming to a head, etc. It's the same as in any comics crossover -- if Daredevil shows up in an issue of Spider-Man, the focus of the issue is still going to be on Spidey and his storylines. If it's a 2-parter that then continues in an issue of Daredevil, then that issue will be more from Ol' Hornhead's perspective and deal with his ongoing plot and character threads. Those titles are there for a reason.