At lower levels, yeah, sure, I can understand how you can fall victim to it. Deep Slumber is one of my favorite spells because it doesn't require you to move or speak, which makes it handy if someone immobilizes you, but it's also a tough spell to justify when you're so low level and have so few spell slots.
But honestly, once you can cast 4th level spells, it shouldn't be such a big problem anymore, because you have Dimension Door, which doesn't even need Still Spell. I made fun of one my friends for quite a while because he fell victim to grappling despite having access to 4th level spells. Really, why the fuck wouldn't you have Dimension Door memorized?! It's Dimension Door! You know what the first thing I do once I can cast level 5 spells is? I pick up Silent Spell so I can have a nearly unstoppable Dimension Door, that's what! It really is one of those spells that demonstrates Bassist's point about how Mages really, really take off around the 4th and 5th spell levels. Before then, you're a balanced character that is really nasty in short bursts. After you get it, you can just start being a complete douche bag to everyone and have the tools to at least get away.