I run two regular water bottles, too and one aero bottle with carbs in the frame triangle.
Built in hydration systems are super nice, but maybe not so useful in IM races. The litter zones would be too short for me to grab 1-2 water bottles, squeeze them in and throw the bottles in the zone away (or you‘d have to brake and slow down even more…). Sometimes the bottles are not screwed up well enough, so you‘d need even more time.
For emergency fueling if loosing bottles or failing to get bottles during aid zone the built in systems would be very nice.