There was a time when Wikipedia articles on palaeobiological and geoscientific topics where almost exclusively abysmal, including unreferrenced children's-book-like fossil animal portrayals 30 years behind the state-of-the-art.
That there has been a change during the last years, is more or less the consequence of the infusion of expert or semi-expert knowledge. Compare this and this version of the English Wikipedia article on a famous fossil taxon and you may acknowledge that a 5 year riping process can result in a nice and relatively up-to-date encyclopedia article.
Scepticism from the scientific community had different causes:
(1) unreliability as a data source/ lack of references - these problems were mostly solved when the community of WP writers established higher standards and agreed on that all major points made by an article have to be supported by independent sources.
(2) WP-related plagiarism by students. I suppose this is merely a problem of teaching youngsters the difference between right/honest and wrong/inhonest use of source texts and nothing Wikipedia can be blamed for.
(3) potential influence by people with a special agenda. Mostly neutralized by the overload of sensible/ unbiased Wikipedia workers. In most cases the WP community has enough background knowledge - so that unsupported/biased statements are recognized and rejected/presented in a neutral way.
There is also the question of utility, especially from the expert's point of view...
Why writing WP articles can be a worthwhile operation...
(1) Your contribution will be read. Search engines mostly place Wikipedia articles very high on the list. Anyone searching your article's lemma will find your text and use it. Thus you will be the provider of primary insight into a specific scientific topic.
Your article may be helpful even for serious research - namely for scientists of related fields searching for the explanation of a term (and references to further technical literature/ online sources).
(2) You can bring rarely publicised aspects of your science to public awareness by the choice of the topics you are writing about. By creating a featured article on a rare species of ammonoids you can direct the focus of paleo-enthusiasts to an otherwise neglected field (you gain more publicity for what is really at the heart of your interest).
(3) Sometimes it is a good excercise to recount for a non-specialist audience what appeared plausible in the internal discourse among scientists of your field. You may find that some concepts are hard to explain (and perhaps don't make much sense when in the cold light of day...).
This list could be could be continued.
Hindrances for new expert authors?
Frequent misunderstandings about the Wikipedia project are listed under "What Wikipedia is not" - you should read this to prevent later disappointments.
There are some Wiki formats - mostly as easy or easier to handle than HTML codes - and you are facing a growing body of rules and conventions. A recipe is to copy and paste the style/format features of an existing article code and to fill in your new content.
Don't argue too much about styles and formats with established Wikipedia users. Your primary aim as a specialist should be to provide up-to-date information, scientific background and overview knowledge, and corrections on misconceptions/ misunderstandings which often arise from the popularization of science.
If you out yourself as an expert you will find that amateurs among the experienced Wikipedia users will give you much support at the beginning and back you up in discussions (which can arise if you are working on controversial topics).
My own Wikipedia experience
I had my first edits in the German Wikipedia in 2005 when I was still an undergrad. If English is not your mother tongue you may think about editing articles in your own language WP, but mostly in the en:WP there is more substance to start with.
In any case you will find that much work is to be done and that you have to be selective with your engagement. Look at the points that you believe are most important and easily done. If you have done a literature search for your professional work you may use some of the data again for a WP article. If you are a PhD student, be aware of the Wikipedia procrastination potential!