As a marksman for example, when I'm doing great and dealing high enough damage to contribute to fights in a meaningful way, I can afford buying some mixed items (Frozen mallet for example) to avoid being blown up instantly were I to take one step too many toward that one assassin. That makes it rather difficult for the enemy team to shut me down, and in turn lets me consolidate my lead. My damage's not increasing as much as if I went for purely offensive options isn't really a problem, as long as it's enough to do my job.
As a marksman again, when I'm not doing that great nor dealing high enough damage to be a serious threat to the enemy, and I build some mixed items, I can make it so that it takes more time for the enemy to get rid of me, but if I'm not a threat anyway, what's the point? I'll put myself a little farther away still from dealing enough to kill anything, and if I can weave even two more auto-attacks before being killed off by the assassin, what difference does it make? If I picked an Infinity edge I could at least hope for one or two lucky critical strikes to shut down a squishy opponent, but with a Mallet or such I'll just be dealing shit for damage until I'm wasted.
From my limited experience, the choice is as follows: while ahead, either you go full-on offensive and risk getting blown-up by chance, or invest in some defences to prevent your enemies from stopping you; while behind, either you go full-on offensive to try your luck at shutting down an enemy, or invest in some defences to be useless for the rest of the game. Notwithstanding the fact that completed, defensive items are pretty bad investments since they cost more than they're worth.
Briefly put, if you're getting blown up by your opponent, there isn't much of a chance your buying health will help you.
