istemek is a peculiar verb. It roots from a verb meaning ´going after, following´. But in present day language just like it was not one but three or four verbs:

1. to feel desire

Birşey yemek istiyoruz.

2. to ask, to utter a wish

When mother said grain is finished Keloğlan answered: Komsulardan isteriz.

In this sense istemek can also be used in the imperative form: Hukümetten yardım isteyin.

3. to be necessary

Yurdun ilerlemesi için çok çalışmak ister.

Does it work in other persons also or has it developed into an adjective like gerek?

4. in idioms and sayings also something like ´take which ever of the alternatives´

İster sağ ister sol, yeter ki insan ol.

İster misin as a sentence adverbial means just about the same as ya olursa ´what if´ while it still has the concrete meaning also.

Confusing enough.

Good analysis Abla. 

3. to be necessary

Yurdun ilerlemesi için çok çalışmak ister.

Does it work in other persons also or has it developed into an adjective like gerek?

it is also used for to be required something which is the same thing as "necessary"

requires

Türkçe öğrenmek sabır ister. [ Türkçe öğrenmek sabır gerektirir]

Learning Turkish requires patience. [ Patience is necessary to learn Turkish]

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İster sağ ister sol, yeter ki insan ol.

whether you...........or .......

The lyrics of a Mavi Sakal song plays with this ambiguity (and others, too).

İster misin bulut yağsın gökyüzünden

yine de ister misin?

İster misin ölüm yağsın söz yüzünden

yine de ister misin?

Diler misin demir yağsın ön yüzünden

acımadan ister misin?

İster misin yıkılsın tüm köprülerin

yine de ister misin?

It is impossible to translate it literally, so I just let it go. My Try:

What if it rained heavily from the sky

are you still along?

What if people died in great numbers because of a word that has been said

are you still along?

What if it rained iron because everyone wants the pole position

are you still along with no pain of conscience?

What if all your bridges were ruined

are you still along?

We had talked about 3rd person imperative being used for condition here:

http://www.turkishclass.com/forumTitle_52268

See for example post # 8.

We see the same usage in the lyrics.

So for example:

İster misin bulut yağsın gökyüzünden

Would you like if clouds above rain (or turn into rain)

it is also " needing" 

Bu ev iyi bir tamirat istiyor. 

This house needs a good repair.

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Sometimes , in colloquial Turkish " İstemez ! "  third person singular aorist form is used for the first person singular as an impolite negative reply [No] when we are offered something or we are asked to do something. We give that impolite reply  it is because whether we are in a bad mood at the time or we got angry or offended with that person.

A. Çay alır mısın ?       ----------> Would you like to have some tea ? 

B. istemez                    ---------->  NO

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A. Otur da dinlen biraz. ---------->  Have a sit and get some rest.

B. İstemez                     ----------> NO

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It seems that the old *izde- ´to follow, to search for´ (OMG is it really iz there) has both lexicaliced and grammaticalized. The lexical development is meaning 2, ´saying loud what one desires´.

Grammaticalizing into more abstract meanings like necessity has gone through getting accustomed to different types of subjects i.e. subjects whose referent is inanimate or in the next step, infinitive. That has opened a road for new pragmatic interpretations.

I even drew it but for some reason I cannot copy paste a diagram from Word to the Add Message box. Haha who cares anyway.

Thank you tunci. And thank you si++ for reminding that special use of subjunctive  -  certainly these sentences are examples of it.

Abla, where did you find the verb izdemek? It is still in use in Kazak(izdev), Kyrgyz (izdö and Uyghur Turkish(izdimek).

This is the istemek article of Nişanyan sözlük:

www.nisanyansozluk.com

I am sure you will find it very amusing, Umut.

Ah i know that sözlük. I thought you had a different one.

We are really lack of etymological dictionaries.