Consider this: Improved familiars can have hands with opposable thumbs (Imps, quasits, mephits) and can therefore use its masters skill ranks rather effectively when compared to a rat or a hawk. Also, an such 'imp-oid creatures' can talk, making it able to use UMD on the items that it holds in its hands, effectively giving you an extra spell per round. How is that compared to +3 on your spot check? Other improved familiars make vastly superior scouts when compared to the standard familiars, especially air elementals, that doubles as the best way of delivering touch spells, and earth elementals that can literally poke its head through a wall and tell you whats on the other side. Other familiars can serve as mounts (hippogriffs comes to mind), and others again are just plain cool (hellhound? YES!).
Standard familiars certainly have their merit, but don't write off improved familiars just because they cust a feat and cost you a minor skill/save bonus.