You say you want to build a website? It must be feature rich, flexible, extensible, powerful, and very web2.0. This is an important site, and you don't want to be locked in to someone else's framework, so you have decided the smartest approach is DIY. You have a small team of very experienced LAMP developers who have track record building successful sites. They've promised to meet your every requirement. You're new site will look and feel unique. It will not have that distasteful look of so many of these other sites built on one of the open source content management systems (CMS) like Joomla, Wordpress or Drupal. Down the road, as you extend your website, you will save lots of money as you won't have to hire developers with specialized skills in a certain CMS. Anyone with sufficient knowledge of Apache, PHP and MySQL can work on the site. A CMS might work wonderfully in many situations, but in your case, you need the power and flexibility only offered by a DIY solution.
Sounding pretty reasonable to you? If so, you are probably buying in to some common fallacies.
Using Drupal (or any other CMS) does not result in any more 'lock-in' than a home grown solution. The fact is, once you've developed your site, you're pretty well committed to whatever solution you chose unless you're up to completely re-developing the site. My question to you is which would you rather be 'locked in to', a well known, used, tested and documented system such as Drupal or something crafted up in the last six months by your team which likely has no documentation, little testing and almost certainly no architecture for easily extending functionality as the demands of your site evolve?
Smells Like Drupal
I regularly hear folks talking about 'the Drupal look'. Drupal does come with several default themes that are each distinctly Drupal. Most websites either stick with these themes or variations of them that don't go out of their way to hide the Drupal roots. So, yes, most Drupal sites look like Drupal. That is not to say that using Drupal requires that your site look like Drupal. Drupal is completely themeable. Most visible aspects of Drupal can be customized using plain old CSS (ala CSS Zen Garden). If you truly want to tweak the details of element rendering, you can do so with Drupal's cool PHPTemplate system. At the end of the day, your site can look as un-Drupally as sites like The Onion and NowPublic.
Special Secret Knowledge
Another favorite of mine is something along the lines of, "we want to develop our site in pure PHP using open technology and standards". Yes, I've heard that more than once, I'm not making this stuff up. I won't bother pointing out to my savvy readers that Drupal is written purely in PHP and relies only on equally common and open technology such as MySQL, PostgreSQL and Apache. You already know that.
There is validity to the argument that developing a website using Drupal will most likely require some degree of knowledge of the Drupal API. If you want to create functionality that doesn't already exist in Drupal or in one of the many third-party modules, you'll have to learn how to write Drupal modules or find someone to do it for you. But there are thousands of Drupal developers worldwide. You'll have a lot more luck finding someone who knows the Drupal API than you will finding someone who knows the homegrown system your team developed.
In this modest web serf's opinion, building a website infrastructure from scratch is a matter of necessity if you'll be serving billions of requsts a day. Otherwise it's pure hubris. Invest in some time with a good therapist before making any final decisions.