We've got a very attacking* FB in Reguilon, so Tanganga would be ideal to balance the defence a little. Comfortable coming inside and helping out the CBs, and we've seen how much he likes a good block! That's a skill in itself - reading the game, anticipating the shot, and getting his body in the way.
Tanganga does seem a bit lumbering when it comes to carrying the ball up the pitch, but he can do it. You're right in that he's never going to beat a man, but that's what Bale is for. In theory, anyway.
Is he as good as Walker was? Nope - neither defensively (even allowing for Walker's trademark brainfarts) nor attacking. But then, neither is Aurier...and, based on what he's shown us so far, Doherty isn't even on the same scale! Equally, having a Trippier type super-attacking FB on that side would leave us hugely open at the back.
I'm more than happy for Tanganga to be given a run. Good attitude.
* Not to say he can't (or won't) defend, as he clearly can - he did brilliantly against Mahrez a few months ago, for example. Just that his first thought is to attack, which is great.