[lan321]

In my experience it just boils down to:

1- You need a ton of internal knowledge so it doesn't really matter what they know past the basics.

2- Testing gets expensive with seniors

3- You can't get mid-senior level employees you like. I see very often companies having really high requirements for hiring leading to the only candidates passing being friends of employees. Juniors pass easier via the 'he's motivated to learn' path.

4- Juniors bring a motivation with them. Seniors tend to generally care less so a couple of energetic juniors can get them moving a bit quicker. Especially if you find a good one, since a senior really doesn't want to get outperformed by a fresh graduate. Also, since they usually suck at politics, it's easier to prod them about why things aren't working than the seniors who've played the blame game for 20 years and have perfected the art of dodging responsibility.