It might. I don't see how it would hurt to have a page detailing what you expect to be doing when the site is fully functional -- there would be a lot of keyword opportunities in such content, and you'd also be serving any stray visitors who somehow find the site. A contact form geared toward "Would you like to be notified when our site is launched?" is another good, friendly customer service thing to do.
However ... submitting to the crawler-based search engines is all but useless these days. They find sites by crawling the web, not thru sites submitted to them (which are said to be 95% spam submissions). And you don't want to submit an unfinished web site to any human-reviewed directories like ODP, Skaffe.com, JoeAnt, etc. They won't accept a site unless it's fully functional, so skip the submission part.
My wife and I have just launched a P/T home business. The home page for this business has said nothing but "stay tuned" for the past 5-6 months. Somehow, Google found and indexed that page. It was never submitted, it's not linked from any other sites ... nothing. I have no clue how it got into Google, but that's another thread some other time -- the point is that your Welcome/holding page may be indexed without having to submit, and once it is, then you just have to wait for the crawlers to revisit after the fully functional site launches.