equipment recommendation - Would a prime be redundant with a fast zoom?

Now that I am finally getting a fast zoom (Tamron 17-50mm 2.8) I've been considering a fast prime to go with it, specifically a Sigma 50mm 1.4. Despite its shortcomings, it still does really nice subject isolation past f/2, which is important to me. I was wondering if it might be redundant since my zoom is already a pretty fast fixed aperture. Of course, 1.4 is two stops faster than 2.8, but as with all primes you have to stop it down to get sharp results.

The main thing I am worried about is that if I drop $500 on a Sigma, it won't get used because there is already something O.K. in that range. Basically, is two stops difference enough to make you want to switch lenses?

EDIT: Perhaps I should add and as may have already been mentioned, these $500 could go to a nice speedlight which could sove the low-light 'issue' you get with f/2.8 vs f/1.4.

Answer

f/1.4 is very useful if your other lens is f/2.8. I would certainly pull out my bag and grab the f/1.4 lens when the need arises. Indoor portraits, indoor sports, low light anything etc all will greatly benefit, if not require f/1.4. On the other hand, you aren't going to find a 17-50 f/1.4, so that is why you will need the prime.