I give you some eye candy why I personally enjoy playing my vanguard with some grain of the most heard but most seldomly used spice "Rule of Cool".
The copper wire should act as some kind of plasma magazine feed, because why not.
My bikes are very, very decent. Catching some poor fellow with scout bikes and their mortal wound strategem is always nice, but the sheer fire power of so few points is ideal for cleaning all these pesky road blocks. My 3 melta carring bikes often
survive 1st and second turn of shooting. I fully embrace the mediocre nature of Space Marines and let these bikes fulfill multiple roles.
1st: more bodies to shoot at
2nd: more bodies to shoot with
3nd: grab some objective and hope not to die too soon (Objective placement is mostly centered around the midfield)
4th: catch a bullet so some other hero might take down the CURRENT, state of the game most relevant opposing element.
My army does not wipe every army in a short time frame, but it gets hardly beaten down and always plays the Mission. It is like an onion, with layers of boltguns, meltas and plasma.
Your point about delivering metlas with bikes (i also have a Rhino with double melta teams): If i want to stay mobile, and have to, in order to get points why not deliver some nasty shots?
My lack of commitment to staying back with efficient devastators is, other armies can do it better.
The thought is the following:
If Space Marines are mediocre and I stay in the backfield shooting (which they are decent with hiding) I am also wasting their potential to get closer and die not so fast on the frontline while dealing punches and shooting rapid fire boltguns, since hey, its 8th edition and even a boltgun can scratch a Land Raider.
