I agree with everything you said. I already have Pierre on my team, and can't risk having another 1-2 category player hanging around my lineup. It just sucks having borderline dropable players sucking up time on my bench. On any given night I have two outfielders hanging around my bench. This is not fun. I'd almost rather have two solid middle relievers boosting my ratios.
Well, using Grey's projections, Burriss might have 26 or so more stolen bases in him. Kubel isn't a 300 hitter and might have 15 or so home runs left. Statistically it's more likely that Burriss will steal a base than Kubel will hit a homer in a spot start. Both will probably cancel out how they effect your team's average (which in spot starts is practically unimportant).
Depending on the depth of your league, Its probably more likely to find a replacement for Kubel if Burriss doesn't work out than it is for Burris if Kubel doesn't work out.
I'm in a deep league (9 IF 6 OF) and Burriss was the last SB threat but I can name five guys who might go 20/80 by season's end sitting on the wire.