I don't remember which season or by whom, but Joe Mauer was accused of stealing signs at one point when he was a base runner at second base. It was noticed that he was grabbing at the ear hole of his helmet before some of the pitches.
I guess I fall into the camp that as long as every team plays under the same conditions sign stealing is part of the game. By same conditions I mean that no electronic or mechanical assistance of any kind may be used.
But really, if teams don't want their signs to be intercepted they simply have to improve the encrypting of their signs. It's really not that hard to do. For example, you could use different indicators or sequences depending on what the batter's uniform number is, or what the on-deck batter's uniform number is. And this can change from game to game or inning to inning or batter to batter. Even Bruce Pearson would be able to do it.