Hmm... Looking at how Diend summoned Kotaro, my guess is that by right the Riders are summoned when they are caught off guard, while going through their daily routines.
That's why, when he summoned Blade etc, they attacked Decade nonetheless cos they were already transformed and in midst of battle, and had to continue to battle regardless of opponents because they were pissed that they were deprived of the battles they were in the midst of having.
Meaning, if Yuusuke were to do business in the toilet and Diend happened to want to summon him, let's just say the scene would awkwardly funny. =X
(Yeah... I know the scriptwriters just want to pull a Kintaros to make us Black-lovers cry and boost ratings... But hey, just wanna be abit creative =X)