I agree with everybody who mentions what you are planning to work on.
I have 1/4, 3/8 and 1/2 (+3/4 which I got as a deal which will eventually make it's trip here as it is with my friend 100 miles away)
I am a DIY guy who does some work here and there - thus not a pro working on heavy equipment, trucks, etc.
I used to use mostly 1/4 and 3/8. I picked up some extra 1/2 staff when I saw them for a good price on yard sales (have several ratchets, breaker bars, T-handles, most of SAE sockets and some Metric by now (not a complete set though))
This summer I was doing brakes. I am in the rust zone. I could not remove several bolts holding the caliper bracket from inside no matter how hard I tried. I even wacked the SO 3/8 breaker bar couple of time with a rubber mallet (just left couple of dings on the bar, could not even move the bolt). Eventually decided to try a longer 1/2 Craftsman =V= breaker bar. Better leverage and longer size compared to SO F-10-L . Did it relatively easily.
I am also slowly disassembling a very rusty jack (sorry for the ones who saw my earlier summer thread - no progress for now) - I do not even bother with 3/8: 1/2 breaker bar, 1/2 to 3/8 conversion piece and 3/8 sockets (I did not have some sizes in 1/2 drive which are 6 point)
I would recommend the following - if there is a chance you need to disassemble something rusty, stuck or large in the future, find yourself an extra 1/2 set and buy it when you see it for a great price at a yard sale or on CL. The set would not take much space, but can come very useful.
It's better having and not needing vs needing and not having... Hmm, that's how I ended up with 3/4 drive sets which I will most likely never use, but it would not hurt to have them since I got a great price...