I agree, it seems like a natural feature for mobile apps…
It’s possible to do bounding-box queries without specialized indexes; for example (pseudocode) loc.x BETWEEN $x0 AND $X1 AND loc.y BETWEEN $y0 AND $y1. With regular indexes on loc.x and loc.y (as opposed to R-trees) this isn’t very efficient, but it’s workable as long as your data set isn’t too large.