We all know everyone getting this, after zapdos got thundershock it reopened all the old wounds about what could happen if the add signature moves.
Firstly, that doesnt seem likely for a few reasons. One, the reason that articuno didnt get a special move is that theres no ice type even close to it, and moltres and Entei BOTh have their distinct uses for different situation, yes there's some overlap, but one is better than the other in certain match ups, not one's better of the two in all instances. This however wasn't the case for out yellow feathered friend, Raikou outpaces zapdos in all instances, it was given thundershock to try to even the playing field by t closing that DPS gap, it didnt fully close it, but it did help to make zapdos more viable nd justify powering up for those who dont have a raikou.
2, signature moves will likely never be added, lugia's been around 3 times, still no aeroblast, Ho-oh hasn't been able light its Sacred fire, and Mewtwo cant get Psystrike despite being an EX raid for the entire run of EX raids The lati's didnt get their signature moves, and neither did kyogre when it came back. While data for psyco boost and doom desire are in the game master for deoxys and Jirachi, there are several unused moves in the game master (such as dragon tail kyogre, and a few other misc/ examples) meaning its likely they wont be added
Third, even if these moves are added, you have ask yourself is it worth not powering up some of the best pokemon in the series? Is it worth not powering up the best ground attacker or water attacker if you have it for the slim chance a better one may come out a few years down the road? A few legendaries who have sig moves are actually worth powering up because their signature moves are different than their main damage set (ie Dragon ascent and aeroblast are flying moves, so dragon/psychic offense rayq/lugia are un affected mist ball and luster purge are both psychic moves so all lati's can be repurposed to dragon offense, Psystrike mewtwo will honestly have no use other than the odd machamp raid or the occasional fighting type in gyms, as shadow ball and focus blast are what everyone uses for general battles)that leaves only 3 pokemon, kyogre, groudon and ho-oh, ho-oh's trash so forget about that, and groudon and kyogre are good, and yes, their signature moves are strong, but how much better could those moves really be made? Does not powering them up and say missing out on raid rewards and powerful raid pokemon really worth that much? In addition, most lesser legendaries (ie most non box art/game mascots, exeptiosn being raikou and entei) are either going to be out classed, or do outclass other pokemon, but the recourse use isnt worth it (ie something liek moltres vs blast burn Charizard, one takes RC and the other doesnt, so if RC are more valuable elsewhere, the 'inferior' option becomes the better one)
As for nonlegendaries, this paranoia is more understandable, after the eevee event announcement caught everyone off guard (with most expecting ralts for a double fairy set, even though that couldnt happen or my towns expectation of a bagon)and opened the possibilities to other pokemon getting the moves, but I ould like to quell some unease. IT is largely accepted that gen IV wil come out around the eevee event, if not be started by it, an eevee receiving 2 days for is event, and there being 2 evolutions for it in gen IV doesnt seem to be an accident. I know the wording says "only eevee caught during the event will have a special move" but what could very well be misdirection, or as one of my friends suggested, perhaps the move it gets will be a specific type, and that move will correspond to which form it evolves into.
Speculation as to future CD's being legacy moves doesnt seem likely given body slam snorlax is a research break through, meaning in all likelihood, that will be how legacy moves are reintroduced (albeit in smaller quantities, but with better odds for good iv's)
While it is true gen IV adds a lot of pokemon that will outclasss previous ones, theres still a few that are safe bet for a long time
But for the truly paranoid, your safest options for powering up seem to be as follows:
Mewtwo, Latios, rayquaza, dragonite, Tyranitar (both rock and dark sets) Gardevoir, Machamp, Gyarados, Shadow claw gengar, and any previous event pokemon (except blastoise, because blastoise sucks. dragon pulse ampharos will be good on defense because of garchomp)
Possible pokemon that will get events are Beldum, bagon, all starters, so those you'll want to save