Well, the Redeemer isnt a fighter at all, while it should be decently manueverable, i doubt it will be a top of the line fighter, its main role is to punch a hole through enemy lines, drop troops, and provide support for those troops, its not meant to be taking on heavy fighters like the vanguard or potentially the SH (depends on whether the SH is classified as a Heavy fighter or not). Not saying the redeemer wont be able to fight them, just that its not really designed in it role to fight them, I look at the Redeemer and think cool, a space apache crossed with a spectre gunship, crossed with a pelican from HALO.
Oh i have the same issue when it comes to the sabre and the buc, i dont really NEED 2 fighters, but they are miles apart in their roles, and will be good at different things. The Buccaneer with it ridiculous engines is going to flare up on EM and IR scanners like mad. I would not even attempt to hide with that ship, but the sabre is fast, stealthy, and has decent shields, but weak armor, those shields go down, and you are dead if you stick around longer than 5-10 seconds.
The Buccaneer to me, is your Interceptor, it should, feasibly, be fast, and have enough firepower to bring down a larger ship, or at least stop it. Comparable to say the Su-15, or the F-14 Tomcat, these ships are usually well in advance of a fleet, carry enough firepower to bring down multiple targets, and are faster than a lot of other aircraft, not to mention highly manueverable.
The Sabre to me is the stealthy, light footprint. I want to go to Point X, maybe recon, or say something along the lines of a SEAD strike or critical target/HVT it can get you in fast and quiet, take down its target and leave. All while leaving a minimal footprint. Comparable to say the F-22. Its fast, quiet, manueverable, and has a very low signature, with a moderate amount of firepower. Edit: The sabre can double as an interceptor, and get to the enemy with out them ever knowing they are there, but they lack the firepower to bring down larger targets like a connie or a cat or bigger without support from other ships.