Well, they still win their wars, plus they know more about their neighborhood than the competition.
There's no need to keep sizeable forces at any location if one can rest assured that one's ships are fast enough to get there before the enemy does. Which depends on the speed of the enemy as much as it depends on the speed of the defender.
The Klingons or the Romulans would probably be quite unable to penetrate to any appreciable depth before the UFP got enough ships together to slow them down, allowing further ships to gather and slow them down further, etc., until finally the attack would grind to a halt. Not so with the superfast Borg, or with enemies like V'Ger or the Whale Probe that cannot be slowed down or stopped by any amount of firepower!
The Dominion did not have a speed advantage that we'd know of. What they did have was a convenient smokescreen to prevent the UFP from witnessing a slow buildup of forces - the chasm between Gamma and Alpha quadrants, only spanned by the wormhole. Thus, the Dominion could spit out alarming numbers of ships to a location close to the UFP in a short time, upsetting any defensive calculations based on speed and numbers alone.
We know that making do with as few ships as possible is a major advantage: building or operating starships in great numbers is a known bottleneck, as we can see from the fact that Starfleet is short on ships even in peacetime and does nothing to improve the situation. So if the Klingons keep "standing fleets" idled close to strategic spots, but the Feds have their ships do useful work in peacetime, the UFP wins a strategic victory.
It could have easily gone sour though if the Dominion had not given the Feds a few years to prepare. If they had just poured through the Wormhole it would have been game over before the federation could have grouped together.
Plus with Borg out there I would want a fleet of 200 ships at least ready to respond at all times like in first contact. One crew up and the Federation is now the new Alpha quadrent collective