My team is rather low level right now, with my highest levels being level 35 (a Mamoswine I used to beat Clair, and a Lanturn that's been generally useful), and I'm stuck on the Kimono Girls and their Espeon. It should also be stated now that I'm using a Miltank. I decide to go to Route 47 to begin level grinding. The instant I step into the tall grass, I find a Shiny Pokémon- my second on Heartgold (the first being a shiny Tangela I found while level grinding the Swinub). To date, this is the first time I have received two Shiny Pokémon on the one file... possibly on the one cartridge. But no. The game isn't content with this. It's a Shiny Miltank, three levels higher than the Miltank on my team, and knows every move my Miltank knows except Iron Tail (which was taught via TM). ...I'm looking at that Miltank, and genuinely wondering whether the loss of Iron Tail and the EVs my original Miltank possesses is worth the three levels and the awe of having a Shiny Pokémon worth using in a run.
It is always great to catch a usable shiny, I hate when you finally get one and it is the most useless Pokémon like a bidoof. So good on you, catching a Milktank!
If you get a viable shiny, catch and use it. Nothing more fun than to show off your great alternately Pokémon and kick ass.