Those sound like very valid reasons to me... #1 most closely matches my own reason - and the one that many who feel they are "wired poly" would seem to agree with.
While I don't disagree with #3 - which I would phrase as "I can't be someone's everything" a mono might argue that you don't need another romantic partner to meet those other needs - that's why people have friends.
There have been a number of posters recently where your #2 comes into play (and applies, I think, to my MrS to some degree).
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (23+ yrs)
Dude: hetero poly male, live-in boyfriend (4+ yrs) and MrS's best friend
Lotus: poly bi married (to TT) female, "it's complicated" relationships with Dude/JaneQ/MrS (1+ years)
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe