I'm a little afraid that affliction lacks the former (as in pre-MoP) sustained self heal.
the siphon life glyph returns about 0.75% health per corruption tick (might be wrong on numbers, but i tried affliction a few days ago for some battlegrounds and i remember a health return of 3k per corruption tick on ~400k HP).
drain life is not what it's used to be. it's sadly little to never used. one thing that bothers me, is that it not really "drains" life anymore, but is by all means a channelled DoT that does not restore as much health as it damages. It may well be that drain life is intended to be used with soulburn. but even then, it is channeled.