There is no reason not to buy a 9mm (in general). I seem to be in the minority, but I am a big fan of all of the "big three" auto calibers (9mm, .40S&W, and .45ACP), and I'm starting to see the virtues of .357SIG (though I don't own one yet). Generally, they are very close in effectiveness, and IMO all three have certain situational advantages over the others (long term, I would argue that one should have one of each).
As a one size does all caliber, any of the three will do the job. All three are good CCW calibers (in the right gun), good for home defense, and fun at the range. For specialization, I like 9mm best in small CCW pistols and for target shooting, .40S&W for police use and "civilian" CCW in slightly larger pistols (I also think it would be a near ideal military caliber), and .45ACP in compact and full sized pistols at the range and for home defense. I won't get more detailed only because I don't want to get into a "caliber war" and help hijack your thread.
Now, for your specific situation, if I were you, I might wait a little while:
I grew up in a liberal house hold where there were no guns at all and I went from never even shooting a gun to owning 3 in a matter of weeks, and now I've caught a bit of the gun fever
If you haven't been shooting long, you'd probably be better off getting to know the guns you have well before adding to the collection (you have plenty of time to add to it later). Take a year or so, then add.
I also grew up in an anti-gun, liberal family. I was always interested in guns though. During basic training I fell in love with shooting, though it would be about 10 years before I'd buy a gun. My first year I had a .22lr Marlin 60 rifle, then a few months later I bought a Taurus 82 .38spl revolver, and about a month later I bought a Ruger KP89. After shooting the handguns about a year, I traded the Taurus on a S&W 65LS (I always wanted a .357mag, and I didn't believe I'd own many guns, so I traded), and I traded the Ruger on a Charles Daly 1911. For those two years I put many thousands of rounds through the two handguns I owned and thousands of rounds of .22 through the rifle before I started buying more (though I may have added the 30-30 Winchester 94 rifle during my second year of shooting). I think that experience helped me become a better shooter.
If you really want to add something, a medium framed 357mag revolver may be your best bet. You are well covered right now with the SA Ruger Single Six (a .22 allows for cheap practice) and the 1911. If you decide to carry, a full sized 1911 can be carried and concealed pretty easily with a good belt and good holster combo. Carry a little while and then decide what you want for a dedicated carry gun. The revolver however, will give you something you don't have to a much greater degree than any 9mm would. A medium frame DA revolver makes it very easy to understand the differences between DA and SA as it is quite visible and deliberate. A .357mag revolver gives versatility that no other gun gives: easy to shoot .38spl target rounds, decent .38+P defensive rounds, great 125gr .357mag defensive rounds, through some really heavy woods gun .357mag loadings, it can handle virtually any situation. The medium frame revolver (stay away from the J-frames for now) is controllable with most .357mag loadings (if you go with the larger medium frames like the S&W 686 and Ruger GP100, you can handle pretty much any .357mag). Yet, the smaller medium frame revolvers (the Taurus 65/66/Tracker, and the discontinued .357mag S&W K-frames and Ruger Speed/Security/Service Six) can handle CCW duties in a 2.5-4" barrel. With the right belt/holster even a 3" or 4" GP100 or S&W 686 can be concealed, at least with winter clothing. Plus, .357mag is just fun.