Longer period of what her not attacking?
She only has the heal-on-autoattack for the 3 second duration of Skittering Frenzy at the moment. Having it up full-time will provide more sustain over extended fights and clearing jungle camps.
how she is in the meta atm she wont have enough time to auto attack to heal enough to survive
Elise typically builds tanky and isn't exactly a priority target in teamfights. She'll have plenty of time to autoattack while her abilities are on cooldown.
The base heal of the new heal is not really high 4/6/8/10 lol.
The current base heal is 4 at all levels. That's up to a 250% healing increase for her autoattacks with a higher AP ratio. She doesn't have the burst heal from multiple spiderlings attacking simultaneously, but now it will have no downtime.
At 200 ap she heals only 30 hp, 60 with the 100% rappel buff which is risky to use.
You typically use Rappel in fights to dodge, escape, or chase anyways. That's a buff after you take an action you would take anyways.
Besides, considering that the heal is independent of damage done, 30HP per autoattack would be pretty significant. That's higher than Xin Zhao's W (since it takes three hits), Aatrox's W (when above 50% health since it takes 3 autoattacks) and Irelia's W (with W active) when they are all at max rank. Hardly insignificant.