to chime in (unwanted I'm sure

)
yes, this is a legal and commonly accepted tactic for DE (and other lists as well, like Land Raider lists).
all told, it nets you approximately 3.5 inches of gained first-turn projection (board control).
looking at it from an impact standpoint, it really only has a pronounced effect in one of the 3 main deployment types, pitched battle (unless your opponent is wholly unfamiliar with it).
look at it from;
pitched battle- in a pitched battle deployment you will see the opposing DE player set up first (if going second) and can deploy shallower/refuse flank. with the dep zones here, your DE player will have the greatest impact but least maneuvering room if you bubble-wrap/refuse flank. it's strong, but not a game breaker.
DoW- if the DE player deploys the allowed ONE troop(on 'port) and ONE HQ, in all likelihood it's dead in the water before it has any impact...so he/she probably hasn't even deployed (all off-table is best for a DE player here, so it is of no concern for DoW).
spearhead- easiest to deploy shallow and deny the first turn charge, with the potential for two turns of shooting if the DE player still sticks it out in the wind.
if you want to see it from a fairness standpoint, it's simply a function of the game that ALL armies can utilize, just some better than the other...and from the DE perspective (with the loss of 'I always go first if the attacker' from 3rd/4th ed) helps compensate for the fragility of their vehicles...seriously, vehicles you can use a spitball to bring down.
it grew more out of sacrificial forward 'cover providing' units that survive the opposing alpha-strike (when seized upon,
which can be a complete death-knell for a deployed DE player) than any true game-unbalancing meta-manipulation
'I has moar powah' mentality.
it is shocking, and wholly disheartening, if you are caught off-guard by it...but a fairly easily accounted for tactic ONCE EXPERIENCED.
honestly, some of the best DE tactics involve null-deployment...and if full reserves, leads to relatively short/dull games in which one player twiddles his thumbs while the other gleefully grins awaiting his wave of doom...
so for fun, which would you rather have;
a full game, with both players duking it out all 5-7 turns, or
a 3-4 turn game where only one player controls the tempo?
(the latter is far more common if this common, basic tactic for DE is removed)
if you want to 'house-rule' it, that's up to the consensus...but keep in mind, with the 5e functionality wholly developed by the time this 'dex arrived, the units are likely 'costed' with these functions in mind...and any alteration to the core function can overcost these units.