Alli might be a good deal for Newcastle due to the fact that Tottenham will allow him to leave on loan as well. It's a familiar league for him in a situation in which he has to fight for his future and Newcastle doesn't really have a lot to lose by trying.
Their team lacks creativity, Alli has his glimpses and has started the season ok, he might be an addition that won't take a lot of resources and even in his current state, he's still a player that is good enough (quality-wise, effort is doubtful) for relegation fights.
I agree with you. Newcastle need talented players like Dele Alli. And he shouldn't be considered as a bad player just because he didn't find many chances at Tottenham in the recent years. There were some in-team problems and Mourinho didn't have a good relationship with him. He wasn't showing a bad performance in the years he got many chances in the lineup.