There's a reason they call it the "lock screen" even without a password. It is designed to present information while being insensitive to errant touches. It isn't supposed to respond to touch without a specific gesture (swipe of a specific slider or double-tap Home). Responding to a text with the same gesture as unlocking accomplishes this function while retaining the function of the lock screen. If you get a text while your phone is with you and the screen is locked, there is a good chance that it's in your pocket. Between iOS 2-4 I've experienced it unlocking itself a few times even with all of the safeguards it has and no additional sliders or handles on the lock screen. Now we have camera/gallery shortcuts, notification shortcuts, the slider, etc and you are asking for MORE?! We can't have too many sliders and handles on the screen of else it will more easily misinterpret what you want as it touches your leg or as you attempt to extract it from your pocket (often with no knowledge of which side is the touch-sensitive one). The way most apps behave, nothing is actually "closed," even if the state is saved and the process stopped (RARELY when simply responding to a text), so we are arguing semantics. Because you can read the text and decide if you want to respond without leaving (I like that word better) the app, functionality is very similar to a pop-up. The main difference is not witnessing something the app would otherwise present live or having to stop playback of a video or something. Pausing is often preferred and most information you want to see in real-time or when finished are also deliverable in real-time by notification (same as the text).
I tried BiteSMS with iOS4 and hated all the glitches and crashes it caused. I'd rather deal with the other quirks and use improved iOS5/6 messaging. Of course, I've been on Android exclusively since 2011.