I have not experienced all weather planes but generally they are supposed to add both gameplay and realism. The idea gameplay-wise is that if you're going to be sailing around enemy controlled waters positioning yourself to sink their vulnerable merchant ships you better do it at night or during bad weather if you're willing to burn the extra fuel, planes can't fly during those times allowing you to move around however you like intercepting enemy ships or convoys. This is also realistic as historically submarines always dove in the morning in enemy waters. "We dive at dawn" is an accurate mantra for how a sub should operate, that is their ace in the hole ability that no other ship has. During the Tokyo express runs in the Guadalcanal campaign, for example, any Japanese ship left in range of Henderson Airfield during the day would've have been bombed, so what the Japanese did is they transported troops on fast-moving destroyers rather than troop transports to reinforce their position on Guadalcanal under the cover of night, and were safely out of range of bombers again before dawn came. Submarines were also used for this purpose on both sides. You have the sole ability to exploit the plane's only weakness (night) at the cost of movement.
In fact I'd even say your main enemy as a submarine is planes, not destroyers. Submarines' purpose in the war is to counter planes' control of the seas. You have the unique privilege of sailing around enemy waters because of your ability to dive. Any task force or even a lone torpedo boat can go and sink merchants and rack up the tonnage but because planes exist they would be detected and bombed before they sailed around for long. This is why airfields meant control of the surrounding sea.
Yes sailing around during the day constantly diving when waves of planes come after you is annoying but the solution is not to reduce the number of planes; but rather to not be surfaced during the day and do all your traveling at night. That way you can keep time compression on and simply surface and sink whenever dusk/dawn comes, not very time consuming at all once you get used to it.
Taking away planes is not a solution but merely a "I want an easy and non-challenging way out" rather than a realistic one. Being able to sail 24/7 kinda makes intercepting convoys too easy and simplistic. There's no other factors besides "it'll take us both this many hours to make it to point C, cool let's go"
This is not to say daytime sailing is not impossible, just a risk depending on when and where you are. In stock I believe it's not even a risk at all, just a slight annoyance because in stock your air radar tells you when *any* plane gets close to you and gives you adequate time to dive. While playing the mods I have now (RFB and RSRD) I've on several occasions gotten surprised bombed and sunk during the wee in between hours even with air radar because of undetectable low flying bombers.. certainly frustrating, but ultimately my fault and avoidable. My point is they are meant to provide a real threat to sink you, not simply interrupt your time compression. Planes are bad news. And in RFB one bomb is all it takes anyway
Be wary of when you're several timezones away from your base though, morning can come sooner than you think if you're not paying attention...
