I can't help but think that those who see Son as an invaluable player who should be in our first eleven are people who only watches highlights.
Yesterday he did in fact offer more than another selfish outlet, he did involve himself defensively as well as offensively, and actually did deliver a good cross that could've been an assist and some decent teamplay. But in most of our games he's been running around seemingly unaware of the offside rule and of his team mates presence.
If he'd learned how to look for and pass to team mates he could've been a truly top world class player, but he's mostly just a flashy, technical speed merchant with good finishing. The description may sound awesome, but doesn't fit all that well in a Pochettino Team.