Patkeefe, the reason you want to run both is because they are complimentary to each other and they both serve a worthwhile purpose. Running without a blowoff valve or bypass valve of some sort is a bad idea as it will slowly damage the turbo over time. How can it damage the turbo? Well basically when you are on boost and you lift off, you'll get compressor surge where the compressor basically stalls due to backpressure in the intake. This can cause excessive wear on the center section and eventually the turbo can develop more shaft play among other things. On something more expensive like a 35r, I'd definitely be using either a BV or BOV.
As far as bypass or blowoff valves affecting spool, they definitely can. If your BV/BOV is leaking then it basically causes a boost leak (well, leaking back into the intake that is, if recirculated) which can lead to increase spool time between gears among other things.