I can pop down to the local phone shop and pay literally £10 for a simfree (If not just pay another £10 for pay as you go ) phone which is perfectly capable of watching videos. (Running Android 4.4: http://www.gsmarena.com/alcatel_pixi_3_(4)-6933.php )
While that's fine and dandy, if you happen to live in an area where phones that cheap are a thing, there's a lot of places where that's certainly not the case. Either way, lugging around another device isn't really a good solution when we already have a device that (at least in theory) should be able to play some of the more simplistic media formats (think mpeg, or maybe xvid, if scaled down to proper resolutions).