Its a tough call. Honestly I think it comes down to "Is it worth the time to optimize every page?" I would say yes, if you have the financial ability to do so. I don't think that Google looks at pages individualistically, and looks at the entire site as a whole. If you increase the entire quality score of your site, that will help you more than optimizing a few, or even in your case a few hundred pages. Chances are your competitors will not be going through the same amount of SEO work, and your site ranking will improve. We have tested this theory on many ecommerce sites hosted by us, and seen excellent results. Many smaller websites out rank the leaders like Home Depot, Ebay, Amazon, etc consistantly.
For your meta descriptions I would do something more like:
I know Matt Cutts states that meta descriptions don't have an impact on ranking, but I just flat out don't agree. Depending upon what you search for, you can clearly see the bolded and highlighted words in the link URLs, description, page title etc. Look at the bolded words in the sample above to think of what the description covers.
1. Childrens face paint
2. Halloween face paint
3. Black face paint
4. Face paint
If you so choose, you could also leave the descriptions blank, as long as your on-page descriptions absolutely rock. One rule we always follow internally: "You never know where Google will pull content from, so make sure anywhere they can is awesomely optimized" This would include titles, meta, on page, keywords, everything. Hope this helps!