If you want single-speed, the reasons for doing it properly, rather than simply leaving it in one gear, are mostly about it looking nice. Fixed, however, is a quite different riding experience. If you don't want to experience what it's like not to be able to coast, and to be able to slow down with your legs, then there's no reason to do it. If that sounds interesting and potentially fun, then the only way you can do it is by converting to fixed.

I have a single-speed road bike. It has no particular advantages (small weight savings, and chainline aside) over a multi-speed simply left in one gear, but it looks MUCH prettier, in my opinion, plus, when I'm riding, if I'm tempted to shift, I can't. There's a certain freedom to not ever having to think about changing gear, and if you still have all your gears, the thought is still there, I guess. It's not a big deal though. My two fixed gears are a completely different experience. I've posted my SS below - doesn't that make you want to get rid of the gears?