The MLB logo was a marketing thing more than anything else. According to an article in the Wall Street Journal (
The Man Behind the MLB Logo - WSJ.com ), it was designed in 1968 and was to "signify that MLB was placing league-wide merchandizing rights under the auspices of a new umbrella company, and to commemorate the national pastime's upcoming centennial. ."
Of course, now they have a logo for everything. I like that shield as well and I think I'll place it one the sleeves of my players' jerseys. Good find!