This would remove the exchange's invoice amount flexibility just to support a wallet that can't keep its features up to date.
Most wallets that support lightning already support such invoice.

There's a neat feature that allows eXch to adjust the resulting amount based from the amount sent by the user.
This is useful in case the wallet cannot send the initial amount that the user typed in the main page.

But Aqua users may just have to wait since their article has a note that they will eventually support lightning invoices w/o amount, (posted 2 months ago)
here: https://jan3.zendesk.com/hc/en-us/articles/21945913121819-I-can-t-edit-the-amount-when-sending-Bitcoin-via-Lightning