1) Euro rods are usually ~10 ft to allow for extra reach across or up the water. Due to the extra reach, you also will get a longer drift out of it (since the flies track to your rod tip). Think: You want to nymph a soft back eddy across from fast current - will this be easier to do on an 8.5ft rod or a 10-11ft rod if you are on the other side of the fast current?
- Some people don't agree with this one but I find it to be true - IMO it is harder to break off when using a longer rod due to the flex and the fighting butt section - I feel as if it protects your tippet from snapping on larger fish.
2) When you use a longer rod (10-11') in a lighter weight (I prefer 3-4wt) you have significantly more sensitivity for strike detection, being able to tell when you are at proper depth, and more. In addition to this, the longer rods cast mono rigs much better due to their more flexible and sensitive tip. Try putting a mono rig on an 8ft rod and then on a 10ft rod and cast them and you will see the difference in your accuracy and feel. Also tying in with the first question - You will lose drift length and the ability to reach across current. Another big factor is ergonomics, when using a longer rod, you don't have to hold your arm out, you can keep it comfortably at your side (this is how you are supposed to tightline nymph unless you need the reach) -people demolish their shoulders reaching nonstop.
3) I use my 10' 4wt for every river condition. I run a mono rig usually for my nymphing (unless I am on BIG water which I use the rig I commented above). Why? Because I am able to run just nymphs, nymphs with split shot, nymphs with a light indicator (small Oros or NZ Yarn). Best of all, if a hatch is happening and I want to fish dries, I have an SA Infinity WF4F line underneath my mono rig where I can easily just take off the mono rig and place on a traditional leader and boom, we are in business. I really want to emphasize that you can easily fish with an indicator on a tightline system - I do it every single time I go out. Sure, the casting isn't as glorious as other systems, but it works, it catches fish, and in the grand scheme, I would rather sacrifice some casting grace for the versatility I receive.
Other members - if I forgot anything please add on.