A couple of reasons 'why' a date bought might be helpful with larger ports:
1, If I clean my port up from time to time from useless profit stocks a date bought would be nice to know. Sure, I may not sell PINK2 or Titanic that I've held for 10 years, but it would be helpful (for me) to know it's only gone up $2 in 10 years.
2. Knowing when a stock was bought also can help with daytrading.
3. Constantly checking Trade History (with date bought) is tedious, and only it only goes back so far.
4. Knowing what the % of profit for a stock is MORE helpful if you know when that stock was bought - otherwise, why would I care what the % of profit is, since as you say, "you only care where the price of Titanic is going in the future - the percentage of profit should not matter.
As I said, there's room on our port page why not give traders as much info (that's helpful to some) as possible?
Anyway, it's just a suggestion, your thoughts are appreciated.
Others may have thoughts on it as well...