No major search engines use the meta keywords or categories at all. Don't waste your time. We moved on from these sort of things years and years ago.
Yes, meta descriptions are important as search engines use them for the intro text for each page. If there is no meta description search engines will use the first paragraph to populate the description. However major search engines handle descriptions in a number of ways which can increase click-throughs on your links if you know how to utilise it.
If your page covers a range of aspects about a topic, and you ensure that you include well ranking search terms in each paragraph, then this can be helpful in providing the search engines with a more targeted description to offer their viewers. A search engine will highlight the text surrounding the search keyword to create a description that it displays to the searcher. This increases the relevancy, and the likelihood of your link being clicked on.
So "to write" or "not to write" a description for each page?
In general if your 25 words doesn't describe what the page is about then I'd suggest that it is worth taking the time to write a clear meta description. When I am optimising page with largely static text I create a succinct description of what that page covers because the first paragraph can be less succinct and contain more detail than needed in the description.
The clearer they are about what the content of the page is, the better for the user, and the search engines will be your friend.
Also include opengraph meta information so that social shares will link your data correctly with the correct image and description.
But while important, a meta description isn't much use if it is never seen. Getting your links to rank well usually requires a lot more than writing a good title and description.
Quality SEO is all about quality unique content, good site structure and design, and a first class user experience. This means that you develop your site for your intended audience, you title your pages succinctly and precisely, you lay it out so it is easy to read and to interact with (ie. judicious use of headings and paragraphs) and it should be coded efficiently (in text) without using tricks or gimmicks without hidden links or old black hat techniques. Good copy writing is a major key to good search engine positioning, and good customer conversion. If you're going to spend money on SEO - this is the first place to spend it.
Work on your social integration and social media strategy to create a quality social presence (one that is value and relevant to your audience) and engage your audience regularly.
Pay particular attention to on-page semantics (tagging images, using rel tags and link descriptions).
And optimise your site performance.
Loading speed is very important for achieving quality positioning in search engines, especially for mobile delivery. Reduce images to the appropriate sizes, compress them to the smallest size without losing visual quality. Remove duplicate scripts, keep your stylesheets tidy and pared down, and minimise these files. Use compression and caching and where possible use offsite repository for files - if your site is image heavy, use a CDN. And where possible, setup a mobile version of your site which is tailored to mobile interaction, and which serves images sized and optimised for mobile delivery, and is minimised for mobile.