Bow Marauder build lighting arrow and questions

Ok so this is a variation of the shocker build.
Please give me some feed back.
http://www.pathofexile.com/passive-skill-tree/AAAAAgEBAnEEswUtDH0OPA6tEH8UTRRxFHUWbxjbGS4aOBpsIWAn7Si1KU8ppTIJMok6UkCgQapDnEbXUFBUSVXGVgRW-lfiWGNZ813GXz9gS2CIYVJjp2e9cql0QXTtdPF35XrvfNmCm4Zgi4yMz49GkFWboZ2unrmfy6IApwinhKyqtQS4k7ndwBrGnsbYz2XQ0NSP2t3dRt-_4XPkIudS7DjtIO5v7w7vTvIv8932SPcy-Ov8Tg==

Use lighting arrow with LMP and Life Gain on Hit. I figure eventually I might add chain to it. I am currently level 42 I find for 1 on 1 damage I use poison cloud to take out bosses and lighting arrow for most everything else. I have poison cloud with fork so sometimes I shoot that once first then follow with lighting arrow while the clouds do their work. At this level I am doing fine.
Why do some of these builds not recommend using Greater Multi Progectiles instead of LMP?

Does chain really add a lot to lighting arrow?
I was thinking about using blood magic gem later once I have a 5 or 6 link. With the idea that I could run my auras using mana instead of hp. So purity and most of the damage auras.