It's all about timing and risk tolerance. In a bull market when coin prices are on an uptrend and mining difficulty is increasing, you can accumulate more coins for the same amount invested by purchasing coins directly, but since you only make money if the price when you sell is higher than what you paid, there is more risk. Mining coins allows you to accumulate coins at a lower price than buying them directly, so you have more room to sell at a profit in a market downturn. With a mining rig you can also spread your risk by diversifying in different coins without having to risk more capital. You also have an asset than can be liquidated to recoup much of your initial capital.
If your goal is to diversify in coins by mining and it's on an exchange, it makes more sense to mine the most profitable coin for your rig and trade for the other coins as you get payouts rather than mining a less profitable coin directly.
Currently I mine eth. I was thinking of investing into some altcoins that I think will have some future. I think I can solo mine them too (not sure though).
Looking at current market conditions do you think I should mine those coins or just buy them from exchange. I want them anyway.