I have one barrel built like that. One issue is rain/water getting caught in the bottom lid lip. Also the moisture will drip out of the bottom while cooking so if I use it somewhere Im worried about stains I put a towel or drip pan under it.
I did it for the same reasons you stated, to make the weber kettle lid fit. I thought about taking it apart to get the ash out but with the basket/ash pan I use, very little ash falls to the bottom of the barrel and what does I just shop vac out. I am hoping that over time the "seasoning" will seal the lid to the bottom.
Having the weber lid is great for larger pork butts, I also noticed I tend to get better bark with the kettle lid then I do with the flat lid.