Depends.. If my favorite team is out of market and in the race, I might pay it. I believe similar can be had online. Or a fan of Baseball generally and want to see the games of teams fighting for a playoff spot.
I see that you already ordered this but since you asked about playoffs I thought I should point out that I don't think Extra Innings or MLB.tv provide you access to any playoff games. All MLB postseason games are carried on national channels. I haven't looked up all the details for this season but I think last year the channels that carried postseason games were TBS, FS1, MLB Network, and FOX.
MLB.tv blacks out live broadcasts of postseason games but you can watch them on demand after the game is over. I don't believe Extra Innings gives you any benefit for the postseason. I'm not saying this is necessarily a bad deal. The end of the season is going to have a lot of exciting playoff races to watch. I know I'll be watching on MLB.tv because I'm a baseball junkie. I just don't want you to think you should buy either service for the postseason.
Also, if you have a dedicated streaming device and a solid broadband connection I recommend MLB.tv instead. For $24.99 you can get MLB.tv premium for the rest of the year including MLB at Bat access on your phones/tablets plus access to every game from every team on demand until spring training starts next year.
MLB reduced the price to $50 just before the all Star break.
If you read the terms and conditions, your subscription will auto renew next season at full price.
Next year, the full season price will be around $130 to $140.....
BTW, to my knowledge, MLB.tv does not include playoff games as those are all available on television.