They're not recommended because it's a PITA if you ever have to move your code to a server where it's not supported (and you can't enable it). As you say, lots of shared hosts do support shorttags but "lots" isn't all of them. If you want to share your scripts, it's best to use the full syntax.
I agree that <? and <?= are easier on programmers than <?php and <?php echo but it is possible to do a bulk find-and-replace as long as you use the same form each time (and don't chuck in spaces (eg: <? php or <? =)
I don't buy readability as a reason at all. Most serious developers have the option of syntax highlighting available to them.
As ThiefMaster mentions in the comments, as of PHP 5.4, <?= ... ?> tags are supported everywhere, regardless of shorttags settings. This should mean they're safe to use in portable code but that does mean there's then a dependency on PHP 5.4+. If you want to support pre-5.4 and can't guarantee shorttags, you'll still need to use <?php echo ... ?>.