With that spell available, why would any wizard ever take the silent metamagic feat? I mean usually ppl take this not because they fear a silence spell, but because they want to use spells without being heard.
I disagree.
In my experience, people take silent spell for fear of being
silenced, so as to escape in case they are gaged (gaseous form, teleport) etc.), as well as for being able to cast spells when they've turned into something (transmutation) that does not speak. Especially the latter... This spell does not serve for any of the above.
While some few consider taking silent spells so that they can use spells without being heard (in off combat situations), in the end they get discouraged by the fact that they have to spend a spell slot one level higher than the spell they want... so they mostly don't bother.
Well I know I would have this spell prepared at least twice every day. It wont do much difference in combat (except if you are invisible) but off combat it would be nice. Also, why standard action and not full round?
I've never used it combat. Even while being invisible, i've always had something better to do than waste two rounds so as to cast a spell in silence. Don't forget that a listen check (combat or not) does not allow one to pinpoint the caster, only the general direction (unless he beats it by 20). Imagine that in a combat, with all the distractions and the noise around, its really difficult to know the direction of a single voice... which brings us to the fact that the spell is equally useless in combat... I guess it could find some use in combat, but that would be rare.
As far as the full round question...
I think that the fact that one has to spend two rounds so as to cast something in his silent bubble is enough of a drawback. Moreover it wouldn't really make cense considering the level of the spell.