besides, what disadvantage? you mean the one that you remain braced all the time when wielding heavy weapon? Or the one that you can aim every turn before shooting. The only disadvantage of remaining stationary is that enemies can get to you faster. (they have to engage you in cc, because they cant hit you because of your cloak). Then, when they finally charge you, you can dodge (or get hit in the face), and then on your turn tumble as half action from acrobatics (or simply move as half action, and risk that they hit you) and then obliterate them from point blank range. Of course this tactics only works on enemies with low bs (up to 40) and ws (up to 30). And providing that enemy you are fighting with is not wielding power klaw or something equally deadly.
I can't find any precedent for a twin-linked Flame weapon in the 40kRPG line, although I obviously didn't go through every book, so I personally have no idea, sorry.
In terms of your description above, you're assuming the battlefield you're fighting on is a wide-open plain. There are plenty of situations where remaining stationary is a huge disadvantage: for instance, against enemies that are moving from cover to cover, and remaining stationary means losing LoS. Perhaps the enemy has a mortar set up, and remaining stationary means they can zone in on you and eventually hit you by Blast fire, cloak or not. Any competent melee enemy with Swift or Lightning attack doesn't care if you dodge the first or second attack, so letting them get close is a bad idea. If the rest of the you team isn't playing immobile snipers, then you potentially risk being left behind or out of position.
Anyway, while the situation you described is perfectly feasible, there are plenty of other variables and potential situations where stationary is a bad idea, and a good GM will make it so your comfort zone is consistently challenged.