It's not quite so simple. SDL 1.3 is now zlib (or some similarly permissive license). However, 1.3 is still a WIP (last I checked, at least), and so may not be suitable in some cases. 1.2.x is relatively stable, but is still LGPL (IINM). Whether that's a problem depends on the circumstances, of course.
It probably has the wrong open source license (its unusable as-is on platforms that don't have dll support), and for the platform's you would need the commercial license won't come with code to run on that platform (PS3, XBOX, etc) anyway, which more or less makes me wonder why the commercial license of SDL costs money.
Naturally, if a library doesn't support the platforms you're interested in or if the licensing terms or cost don't meet your requirements, then it probably won't be of much use to you. That goes for any library though, more or less (SDL isn't unique in this regard).
Although SDL 1.3 does provide a wrapper over the underlying graphics API for hardware-accelerated 2-d graphics, many people just use SDL to handle windowing and events, and do all the low-level graphics stuff themselves. As such, SDL is still useful even if you want to write all the low-level graphics code yourself.
Graphics API's in the grand scheme of things aren't all that complicated since the hardware does all of the work, using the APIs raw or even making a basic wrapper for one is a pretty trivial thing to do.
If you're talking about development time, I'd say it's probably easy to underestimate what goes into a cross-platform library like SDL unless you've actually tried to implement such a framework yourself (that is, a library that supports at least Windows, OS X, and Linux, and offers support for windowing, events, basic audio, and so forth). It can be surprising how much is involved (IMX, at least).
For C++ the real time consuming things end up being things like serialization, proper localization support and string handling, and memory management (multiple heaps, streaming textures etc)
Note that I'm not specifically advocating for the use of SDL here; I'm just responding to the points you raised.