Well, I have sometimes this problem with girls, what if they say no? What would people say ?a security hole developers in the code maintained by the 3rd party.. who get the blame?? Who gets the bad press?? I will let you answer...
Seriously now, the developers can always choose what they want and like to do best. It's a free world, but I think the greater risk is taken by not relying on other people's previous work (these people have already hit their head hard against these problems). What I really hope is that developers just sit, relax and analyze these facts, nothing more. Of course, they could say "no, we do it our own way", and that would be fine, maybe not so great for the phpBB community in the long term, but that's life.
And now let's really discuss the arguments:
phpBB developers have to (re)learn and strip down/personalize Smarty repeatedly to take advantage of the focused development
Good point, however my suggestion to use a "stripped Smarty" was targeting mainly the "bloated code" argument. Let's see where we get.
If 'bloated code' were really a problem, one could remove some of the functionality by simply cutting some of the functions, that's clearly an easier job than maintaining your own template compiler. Smarty had 7 minor releases in 2005/2006. The benefits for phpBB team ? maybe not so great, less work, less headaches. The benefits for the community? The same that adodb lite is bringing to adodb.
I do believe that plain usage of Smarty would be OK in the long term. It brings the least headaches and, when phpBB discovers it needs another feature not yet implemented, it'll probably be there. (I have to observe here that conditionals were introduced in Olympus and the need for other such extensions might just come to surface, probably in the same way as these needs came to surface to the smarty guys).