2 Answers

• The webView is the most expensive UI element the iphone and android have.
  To load a webview the os needs to load everything related to( the javascript interpreter, the html renderer, all the logic behind( history, cookies, connections, etc))

  It's a whole safari there except the skin.

  Beside this people tend to put a whole framework( 200k of jquery/mootools/ whatever ) just to make some fancy click effect ( no offence please).

  So in other words - avoid it :) and use as much native elements you can.
  If you need http connection you can use httpclient in an "ajax" style.

• You need the webview.
  For now it's the only way to format text painless.
  And you might want to make an inapp browser.

  I don't say not to use it, I only say DON'T use it for tasks that can be made with native components. And for the rest of the cases try to find solutions to use it in the smartest way possible.
  All you have to do it to think like you have to build a webpage that has to run on 486 computer :D