I've weighed in on this before. As a programmer I'm 100% in favor of timeouts. Most players bet and then jump off to another site or close their browser. (Worldrunner, the software I wrote had a prominent sign-off option, but most players didn't use it.)
The ability to track the number of "active" players at any one time is very important for allocating bandwidth and other computer resources.
If a player hasn't made a play or changed screens for 10 minutes, they he's most likely moved on to another site or logged off. If it is a wiseguy waiting to pounce on a line, then the book has no incentive to make his job easier while wasting even more resources keeping open logins for players that did leave.
The only real obligation a book has to the online players is to provide a current line and then honor bets made at that line. Logins and navigation should work quickly and efficiently. However, since 95% of players won't hit a sign-off button, the software has to make a decision as to whether the player left, and 10-15 minutes of idle time is more than reasonable. (BTW, in the software I wrote, we had a quick "relogin" button for those that had to walk away to make a sandwich or take a call, and hadn't intended to be idle that long.)
The point that WorldRunner made about timeout protecting your account from unauthorized use was excellent.