the point of the LMM is to evolve several different rates simultaneously. If you have rates $f_i$ from $t_i$ to $t_{i+1}$ and take a bond expiring at $t_j$ as numeraire then only the rate $f_{j-1}$ is driftless.
Typically $P_{t_0}$ is used as numeraire which makes all the rates have drift. It generally gives lower variance.
(see my book More mathematical finance for extensive discussion of drifts.)