How often do buy-back clauses effectively get exercised? Genuine question.
If he turns out shite, he'll be sold on to a mid-table team in England or Spain, along those lines.
If he turns out to be at Spurs' level, he'll stay there and they'll have a good player for a decent transfer fee.
If he turns out great, say Bale/Modric levels, he'll go back to Real but even without a buy-back clause, a club like Spurs wouldn't have been able to keep him if he turns out world class.
I can see why Utd didn't wanna go through with it but I can also totally understand Spurs' reasoning for this one.