The slow that Janna has is not very strong, it only takes about 40% of their move speed away. The stalker's blade is to optimize on enemies out of position with a cast slow, this will give your ADC more time to attack or force the enemy to flash. Runeglaive is the only real optimization that makes sense, and it does help with clearing lanes if Janna is solo and has some attack damage, and the utility element is being able to go into the jungle and cast a couple spells and auto attack on the monster to gain a bit of mana back (don't expect to kill them, but when going through the jungle this can be a nice potion or two if you are going to a new teamfight or to place wards)

Manamune gives you a reason to buy a Tear very early, as it takes a long time to get to 750/750, and this is 1000 mana which is very nice, as a Janna with No Manna is just a tasty banana to any mean mamma jamma. You can purchase the manamune before the tear is 750/750, but once this does happen, the 3% of your max mana (2000 to 2500) is around 60 to 75 AD, which is like having a BF sword that gives 1000 mana and 25% regen. This is good for pushing towers, and you also have the option to "turn it on" (use this sparingly, it is very mana hungry) and this would give you a huge boost of 120 to 150 AD, which can really help take down a tower.

If you take manamune before the tear is 750/750, you only get 2% of max mana, which is around 66% of the above numbers, so 40 to 50 AD, which still isnt bad, and it can help your team push towers, as you only have a small opportunity during a laning phase to do this.

This is also useful late in the game, in a team fight, if your team loses the ADC, but ends up Acing the enemy team, rather than have a poor push, Janna can come through with manamune and put some respectable damage on the towers with the golden opportunity.

This is not very viable, if you team needs support because of roaming death squads in the forest, it is far more useful to buy the sightstone and


Why not frostfang if you want cast slow?

Frostfang only gives good gold if you are able to harass, which can be dangerous as janna. The stalkers is a safter alternative to the frostfang, and you can even smite blue when going through the jungle to get a nice amount of mana.

This is played like janna normally would be played for the most part. If you take ADC or Mallet, it is just more important to harass with the auto attack.

Key difference is to try to auto attack every time after you cast a spell to utilize the runeglaive if you have it.

Pros
Tower Push
Runeglaive gives large increase to ability to clear lane minions.
Quick mana regen from jungle. (8% on attack on jungle monster after casting a spell, 1.5 sec cooldown)



Cons
Less room for support items

Only scaling mana today boss.
Mana is Janna fuel, and having no mana is the end of your usefulness. Janna has plenty of move speed, AP gain from runes is negligible, most support builds have decent cooldown reduction.

Runeglaive also returns a percent of missing mana (up to 8% of max mana if you are totally empty), rather than a flat amount. This means it is ideal to go into the jungle when your mana is very low, rather than just half way full, but it can save you a back. This means the more mana you have, the more mana you get back from runeglaive.

Feel free to take Cooldown reduction if you want, it does give more runeglaive, but still, having no mana will kill you and your team more often than spells on cooldown.

Other Advantages to Scaling Mana.
At lvl 18, with Muramana
9x Seals = 21.06 x 9 x 0.03 = 5.68 AD from Muramana (passively)
9x Marks = 21.06 x 9 x 0.03 = 5.68 AD from Muramana (passively)
9x Glyph = 25.56 x 9 x 0.03 = 6.9 AD from Muramana (passively)
3x Quint = 75.06 x 3 x 0.03 = 6.75 AD from Muramana (passively)

5.68 + 5.68 + 6.9 +6.75 = 25.01 AD (just from glyphs) from Glyphs with Muramana (passively)

This can be 200% when you activate Muramana, and this will give you all of +50 AD . Just from Glyphs!
Now thats a free BF sword just from glyphs at lvl 18 with Muramana. This isnt even the main reason, but this sure is a nice kicker.

Compare to AD glyphs.
9x Seal of Scaling AD = 1.08 x 9 = 9.72 (These seem awful)
3x Quint of Scaling AD = 4.5 x 3 = 13.5
9x Mark of Scaling AD = 2.43 x 9 = 21.87
9x Glyph of Scaling AD = 0.72 x 9 = 6.48 (These are even worse)

This amounts to 51.57 AD, which is comparable to the 50 AD from Runes you get by activating Muramana. This build gives you a full rune page of not only mana, but also AD when you turn on Muramana.

Even though there would be no reason to buy the Seal of Glyph of scaling AD over Cooldown reduction or armor, this was just to compare numbers.

These builds can still easily be used without having smite and the stalkers - runeglaive, but this allows you to have a great way to refill mana, with smithing blue and also runeglaive on any monster.