If Gengar runs scared there's a much greater likelihood that it won't last until the endgame when it can cockblock a Persian sweep, since it will actually see use in the battle and hence be exposed to more damage and possibilities to be KO'd, not to mention it's likely to explode at some point. If it gets slept and lasts till the end of a battle it matters a hell of a lot more if you're using Persian because it stands a good chance of waking and messing with your sweep, whereas a sleeping Gengar is basically fodder vs. a standard team. I guess maybe I oversold the importance of the lead match-up though. I really just wanted to share my thoughts about the Gengar ditto for a Persian team, although having Egg para'd from the start isn't ideal since running Tauros and Persian means a team's defensive backbone is a tad thin, but w/e, it's no big deal.
So it is, but you don't have to switch in Egg directly either, you can take a relatively safe route over Chansey on your way there. It's riskier though because you're likely to tank a Snorlax Body Slam or two before sleeping something and it's not guaranteed (though likely) to be Starmie.
Also there's the risk that they'll go to Egg as you go to Chansey. Psychic Starmie lead is well-known enough to make the Gengar switch-out potentially predictable.
If Gengar puts something to sleep and manages to kill off a presumptuous Chansey with a Submission crit (72% damage or so), the recoil doesn't mean anything. Submission is probably Gengar's best "other option," since the rest of his movepool after Hypnosis/T-Bolt/Explosion/Mega Drain/Seismic Toss/Night Shade/Psychic is very bad.
I meant other option as in move that somebody might consider besides everything else that's been used already. Psychic Gengar has seen occasional use for well over a decade.