Who's to say you won't ever be able to buy more alts directly with fiat? Few of the bigger alts can be bought with USD and I only see that expanding in the future.

I'm having an issue on the Realized/Unrealized Gains page. If I uncheck "Add Deposits & Withdrawals to remaining amount" it is showing my ETH balance in the negative. If I turn it on it shows the right ETH balance, but my cost per unit is in the negatives, so it throws off all of my profit values. What is the proper way to deal with this? All of the coins in my balance were ones I recently bought, and did not come in from a deposit, so it should still know the cost per unit.

I don't see a problem with people doing this as long as a few things are true.
1. The sites tell you that they are mining with your CPU.
2. The sites limit it to a small % of the CPU.
3. Normal ad blockers block it from mining.
4. Don't jam your site with ads AND try to slip in a miner.

With that being said, I do not think mining will outperform normal ad revenue in a direct comparison. Even if a site gets alot of daily hits, the majority of users aren't going to be leaving the browser on that site for a long period of time.