I've never purchased mlb.tv, but I do have an NHL GameCenter Live subscription, which I love. (I didn't realize MLB had live games on their app).
With the NHL package, I'm able to watch all (out of market) live games on my PC, phone or tablet (and I've used all 3) on a single subscription, and the quality is excellent. I have my PC attached to my TV via HDMI, and the streaming quality is near HD TV quality.
My advice would be to just make sure you know what you're getting before you buy the package. Last season, for example, with the NHL package, the same subscription could not be used to get live games on mobile devices, which I was unaware of when I bought -- I was VERY glad they listened to complaints and changed that this year!
As far as the "is it worth it to spend money on an online streaming package?" vs. a TV package, well it is for me. My cable provider doesn't even have the various sports packages available in HD, so the online stream is much better quality than I could get through them.
Hope this helps! Go O's! (yeah, yeah, I know...they suck).