Yes - An army without Markerlights of any sort (Pathfinders or not) is completely possible and effective.
I go without them all the time, but it really does depend on what kind of force you are running.
For an Example - Boosting BS.
Troops - I use a Kroot Heavy list generally - so 2 to 3 units of Kroot and 1 unit of the basic 6 Fire Warriors in a Devilfish - The Kroot cannot make use of the Marker Light Tokens, the Devilfish I upgrade to a Warfish and give a Target Array (so BS4) and the 6 Fire Warriors rarely leave the Devilfish unless they are blown out of it.
Elites - My Suits normally have a TL Weapon, and in the case of Deathrains Etc, a Target Array too. - BS4 TL - You rarely miss.
Fast Attack - well its an underused section, Vespid - no thanks -, Pathfinders - Well Pathfinders bring Markerlights to the table but if you dont really need the Markerlights you dont need the Pathfinders. Piranhas I use, but not as an offensive unit - so they dont really need the accuracy.
Heavy Support - Skyray, I tend not to use (but if you do it brings its own Markerlights), Hammerheads - BS4 already and Broadsides - TL rail gun, which I generally find pretty effective without needing a BS boost.
Seeker Missiles - I dont use any.
Minusing Leadership tests for pinning - With the amount of units that are stubborn, fearless etc etc - I simply dont find it that useful (and I carry no Pinning Weapons) - bizzarely the only Army thats really troubled by pinning now is Tau...So if your playing Tau with your Tau, it could be useful :p.
Coversave removal - Granted, this one could be useful, but generally I dont pack that many decent AP weapons to guaruntee removal of the normal save anyway, would be useful against Horde sometimes - but is it worth diluting the rest of your list to squeeze in one ability? Working it out, the extra guns I get from the 160+ points spent on Pathfinders and a Devilfish probablly evens out the overall kill rate.
At the end of the day, we dont use Pathfinders because they are a good well balanced Troop choice (overall - they arent, especially when you have to buy the Devilfish for them [Admitablly 5th ed has helped offset this cost as we can pass it to the Fire Warriors]) - We use them because they are the cheapest way of bringing Markerlights to the Table, the Path Finders without Markerlights would never see the battlefield, but because of the Markers they become quite good. In a Ninja Tau list for example, the Path Finders wont see much use - Conversely however a bare minimum squad of 4 in a Tooled out Warfish can be great, not for the Markerlights - but for the tooled up OUTFLANKING warfish they provide - Armoured support for your Out Flanking Kroot - or if you have alot of Deepstriking Troops, the Pathfinders Devilfish is again useful, for the reroll it offers on the scatter.
But if your list is one that does not need the Markerlights to function, there is not much point in trying to squeeze them in.
Markerlights are Great, they are the Tau's Signature ability - Primarily there use is boosting BS, removing Cover Saves, and Launching seekers - the other options are either defunct or a bit to situational to be really useful.
So if you are running a Fire Warrior heavy list, or have plently of units pumping out high volumes of fire (Stealths etc), If your XV8's are packing Plasma and Fusion for MEQ anhilation, you are running vespid and want to maximise there kills. If you have a deepstriking Army and want it to land on target. If you are packing Heavy amounts of Seeker Missiles and relying on them to anhilate your opponents armour etc etc etc - Then Pathfinders are a great option and will maximise your armies abililities - because they are the cheapest, most effective way of putting Markerlights on the Table. - And in some lists you do "Need" them, because they are a great support unit [thanks to the Markers and the Devilfish abilities] but in other lists - you really dont, and trying to squeeze them into your list will be detrimental to the way your list functions.
So its a "No" - You don't have to use Path Finders to have a successful list, it all depends however what type of list you are playing - like any unit, if they fit into the list you are playing, and maximise the abilities of the units you are fielding - then use them. If they don't, then don't. - You dont have to take them to have an effective force - and there is no point in juggling points totals and dropping units to take a unit that you feel you "should" have if it isnt really going to help you.
They are a great support unit, if you dont need that support you dont need them
