Assuming your TV is compatible with the services and bitrates you desire then there is no sonic benefit to using a separate streaming device. The UI might be easier to use and other secondary features might be better etc.
However it might be the case that your TV is not compatible with every streaming service or some of the hi-res tiers they offer.
Source: Gaming/HT PC, i7 3770K, 16GB DDR3 2400Mhz, 2x GTX 980Ti SLI, MPC-HCx64 w/ MadVR & SVP frame interpolation
Display: Panasonic VT30 55" Plasma ::: Receiver: Denon AVR-1910
5.1 Speakers: Behringer B215XLs
Subwoofers: 4x SI HT18 D4 - iNuke NU4-6000 w/ silent-fans (1 driver per channel at 2ohm)