I don't understand the cynosural jammer mechanics in the context of FW. A Tier 5 system would be a major stronghold, presumably, and people fighting to claim it can wait 1 measely hour for the cynosural jammer to wear off before they bring in their supercapital blob? Not to mention people can just pre-stock capitals in systems they know will become Tier 5 and could be a problem for them later.
1. It makes a number of assets in that system more secure.
Example: You have a POCO or a POS coming out of reinforced at 08:00. You spool up a cyno jammer at roughly 06:50. Attackers will have to use gates, and any fleet they deploy to finish off the POCO/POS will be unable to light cynos during the fight. But since you live in the system (and control the cyno spooling), you can have your cap ships already in place. If the enemy waits for the cyno jammer to wear off, then you'll have repped the POCO/POS already, and they'll have to reinforce it all over again.
If the enemy comes in
before your cyno has finished spooling, that's in itself a tactical advantage. They've played their hand and committed their cap ships to a fight without the possibility of surprise reinforcements. You can decline the fight or escalate to your advantage.
2. It gives the owning group an advantage when beseiging assets in that system.
Cyno fields can be used offensively to cover fleets that are bringing an enemy POCO/POS into reinforced mode. Those fleets can carry on their work without worrying about hot drops, and just watch neighboring systems with scouts.
As an aside, it does give FW groups a legitimate shot at controlling local moon goo, should any be well-organized and motivated enough to try it.
Of course, to do so, they'd have to win occupancy from the enemy militia.