The pros I think are
1. you get the price list you want
2. you start getting some exposure to your website, if your strategy involves getting traffic from Google, then you will be wanting a blog on your website (along side what ever you are selling) as this is a valuable SEO strategy, to start getting content up there
2a. You can start setting up your social network (facebook / twitter / linked in) and refer this back to your own domain
2b. You can start listing your business in free directories, and google places
3. When you start to open bank accounts etc you will have everything in place
4. you will be more confident when people challenge you in a couple of months 'how long have you been in business' you can say a couple of months, not a couple of days
5. you will be confident in your company name and registration number, so you can work on any stationery you might be considering (biz cards / letter heads) in advance
The cons
1. your company year end (and hence account filing) will come two months earlier - but that said - most accountants would adjust your company filing date to 31st March even if you set up your company in May, as it makes the company year / dividends etc much neater. (if you set up your company 1st March 2013 I think it will automatically be set to 31st March 2014 - so in a way this is a pro

)
2. if you currently are employed and you contract of employment requires you to declare any third party directorships, then you should in theory declare
Can't think of any other cons