I don't think it has anything to do with 25K restrictions. I think restriction only applies to equity markets, not futures. Here is the link to rules
http://www.interactivebrokers.com/en...DayTraders.php
Combine accounts give a convenience to spread trade over several accounts. instead of setting 3 trades in each account ( 3x open trade + 3x to close it), you just put 1 trade ( 3x size and it is automatically divided between accounts).