Are O-scopes necessary...

No. Not if you are willing to bust your ass trying to fix simple problems !

Our service department (industrial controls) fixes stuff without scopes. 
They tear apart a working unit and swap modules until they get a fix. 
Sometimes they blow up the working unit too... then an auto assembly 
line is down. They call us up crying. Our first response... "Do you 
have a scope?"  Usually the answer is..  uh yeah... out in the car/truck/
whatever....

The signal tracer ideas are really good BTW.  If I don't have a scope I 
try that.  Its OK for audio amps etc. Fixing high impedance stuff is 
another story. Either the loading or the capacitance kills you. SOME 
circuits will not even work if you touch them with a scope probe !!! 

Get a scope... get a life.  I remember when I was buying my first 
computer... on a payroll deduction loan.  A fellow engineer suggested 
that I tell my wife that the computer will pay for itself out of "increased 
future earnings". He was FVCKIN-A correct. My salary jumped the 
price of that computer that very year. Knowing how to work the tools of 
the trade will increase your value to an employer.

And get to KNOW how to use the scope. Its not just pretty pictures. Its a 
window into a world we do not usually see.  Expecting an oscillator
and have a flat line? Expecting DC and have oscillation ?  Your "audio" 
range listening  devices will not help with ultrasonic oscillations !!! 
Your meter will not help you. Your logic probe (very likely) will not help 
you.  The chip you just put your finger on is burning hot...

Why ???

Get out the scope !!!  Buy beg borrow or steal one ! Get an old one... 
whatever. Why bust your own ass willingly. You will buy one once in
maybe 10-20 years ! So $200 today is what... $10 a year ???  You 
blow that on booze and women in one night   (well... if you are a "big 
spender"...  that is...)