I'll never forget that scene with the Borg cube approaching Earth, in BOBW. Three missile type craft are launched against the cube. Just three. And I think they are demolished somewhere near Saturn. Now, I can appreciate avoiding the cost of keeping a fleet on patrol full time around the Earth solarsystem, when the odds of attack are very slim. But certainly a dense matrix of fully automated defense installations could be stationed in various key parts of the solar system to deal with threats. It never struck me like the Federation was hard up for resources. Quite the contrary, the organization seems to be quite flush with them. Why not devote a small portion of that towards precautionary defense measures?
I chalk up the lack of defenses in BOBW to production costs and timing. They devoted a big bulk of that to Wolf359. Otherwise I think they'd have shown much more.
What is a small portion, a cruiser squadron training cadets and resting or just the newest ship coming out of the shipyards getting ready for her maiden voyage?
How many Federation planets are there now and can each planet have a squadron of cruisers? As the main proponent of all for one and one for all thanks to Captain Archer, Earth is the last planet that can politically put up an independent guard and has to show confidence in depending upon Starfleet's in depth defenses.