the posts (and topics for the titles) tables are scanned and the words in the posts will be looked up in the wordlist table, then one line for every unique word in the posting is inserted in the wordmatch table (according with the number of times the word occurs in that post) and with a flag that indicates if this word is present in the title of the post.
note the last potion of that statement
there is a flag that specifies if the search word was in the topic of the post
this works well for the forum since each post consists of a topic and a text
but this does not work for alot of other things
-> a portal needs a more flexible searchsystem
the same will hold true in alot of other places, where a portal needs to be more flexible than a forum
this can of course sometimes lead to necessary changes to the forum and probably in a slight performance decrease for the forum (its hard to be the best at everything)
so if you want to easily searchenable every new module you will have to change the searchsystem (will the phpBB team be ok with this?) -> the forum will be nothing more than a really good and important module within an application framework (even if the application framework has taken most of its core components from the module at the start)