more sense than Bale, sure, but Alba? a Catalan La Masia graduate who would take about five seconds to settle into the team AND would cost less? no.
and look, Tabarez plays Maxi Pereira at RB, so he has a tolerance for attack-happy fullbacks... but does he let Alvaro near LB? does he trust him to play a balanced role? no. he plays Caceres there and lets Alvaro run amok up-front. that makes me wonder about his defensive qualities much much more than 'Arry and Gary Speed (may he rest in peace) playing Bale as a LW.
Tabarez uses like 4 or 5 different formations though, Sometimes he uses Alvaro as a wingback, in a lot of instances he uses Caceres on the left and Alvaro in front of him, but in that case Urugauay is often playing a slightly off-kilter 3-4-3 with Maxi a bit deeper. But part of that has to do with the fact that Alvaro is a more multi-dimensional attacker while Maxi can only run in a straight line. Alvaro is not exactly a guy who will bring defensive responsibility but he plays LB plenty of times, and in Uruguay there's shifting roles so it's hard to judge absolutes from that.
Oh, and technically Alba isn't a "graduate", he was basically a la masia reject who the club gave up on when he was 16. He was also never a fullback at Barcelona, he was trained entirely as a winger until Valencia decided to convert him. I know your theory that every Catalan who ever stepped foot in the Barcelona youth ranks has a mystical connection, and he's no doubt a very technical player familiar with the league, but we are talking a guy who was a fringe prospect until Valencia saw something in him.
And there's also the fact that Alba is a slight player. Pereira is not an intelligent defender but he has tracks back and the physical edge makes a difference.