I've got two smart tvs. Both suck at being smart and both turn themselves off (likely due to overheating) with prolonged usage of the apps. The apps on both are very lacking compared to the ones of my Roku or Xbox. That may not bother everyone, but the functionality is terrible (yours may be different).
As for other reasons to get something aside from the smart tv - My roku allows me to do a lot of things that the tvs can't. Normal users may never use any of those options, but for me it makes a big difference.
With cutting the cord though, I can certainly see the savings. I don't have HD cable right now. I get channels 2-64 on cable and it's over $50 a month and it's only that cheap because I bundle it with my internet. You can get Netflix and Hulu Plus for $18 a month. I've also got Amazon Prime (I paid $72 for the year, but it's normally $99 a year). I don't know how to price that out because I certainly use it for more than just the video service (free 2-day shipping on orders through Amazon, which probably happens once a month).
Now, if I were to get MLB.tv then we are talking about $20 a month - but that's still cheaper than the upgrade to HD cable. Of course, the TWC app on the roku/xbox gives me about 40 channels in HD, and don't tell them, but it even gives me several channels I don't get on my TV, so I often watch stuff on those apps because well, watching non-HD channels is the worst and I hate it.