Bad Habit or Just Pointless?

If it's comfortable for you then it's working. just make sure there's no pressure put on your thumb and you're fine. maybe try just touching your bass with the side of your hand and not the tip of your thumb. It's something I tried out a while ago and let me move around a lot easier between bridge and neck

 

Plenty of players have made it doing just as you are.
If you want more variety in your playing, you should experiment with playing close to the bridge also.
The strings are much taughter there and you might be able to do quicker passages more easily with a little practise.

 

I actually wrote a paper on this once. the best thing is to practice without your thumb on the bass at all and practice using the tips of your fingers and the fat parts and then decend your thumb back onto the bass in the spot that feels comfortable and maybe it'll change. I find the more straight up and down my hand is the quicker I can play and the easier it is to articulate. I like my fingers and thumb to look like I'm holding a deck of cards. that works for me

 

I don't know that it's necissarily bad, you'll just get a really round sound, moreso than if you were closer to the bridge. I know that when I play my Warwick, my thumb rests on the middle pickup, but the fingers are over the bridge pup, and i've never had any problems, so I'm curious to see what others think.